Magmatic evolution of the southern Coast Belt ...

Magmatic evolution of the southern Coast Belt: constraints from Nd Sr isotopic systematics and geochronology of the southern Coast Plutonic Complex1 Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/31/15 For personal use only.

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R.M. Friedman, J.B. Mahoney, and Y. Cui Abstract: Igneous rocks of the southern Coast Belt (SCB) and adjacent Insular Belt developed within a Jurassic-Quaternary magmatic arc built across accreted juvenile-arc and oceanic terranes. SCB plutons are mostly of intermediate composition, with I-type characteristics and major element, trace element, and rare earth element geochemistry consistent with genesis in a subduction-related magmatic arc. Ubiquitous xenoliths and migmatitic zones at pluton - county rock contacts indicate that assimilation of crustal rock was an important magmatic process. U-Pb zircon crystallization ages for SCB and Insular Belt igneous rocks indicate an overall eastward migration of the magmatic axis from Middle Jurassic through Late Cretaceous time. Although absent in most rocks, traces of old inherited zircon are present in several Middle Jurassic - Upper Cretaceous plutons in the southeastern Coast Belt. The primitive character and restricted range of Nd-Sr isotopic data for Middle Jurassic to Quaternary igneous rocks of the SCB (E,, = +2.4 to f8.0; Sri = 0.7030-0.7042) indicate they were generated in an isotopically juvenile magmatic arc. The distribution of isotopic values along the mantle array and the wide range of A,,, values suggest magma was derived from depleted mantle within a mantle wedge, with little or no contribution from old, isotopically evolved continental material. Although field evidence suggests that assimilation of juvenile crust was an important process during magma ascent, isotopic and geochemical data do not permit discrimination between direct mantle derivation of magmas followed by fractionation and crustal assimilation, and wholesale melting of mafic arc-derived lower crust. RCsumC : Les roches ignks du Domaine c6tier mkridional et du Domaine insulaire adjacent se sont dkeloppkes, du Jurassique au Quaternaire, au sein d'un arc magmatique construit au travers un arc juvCnile accrCtC et des terranes ockaniques. Les plutons du Domaine c6tier mkridional posskdent gknkralement une composition intermaiaire, prCsentant les caractdristiques du Type-I et une composition en terres-rares, klkments majeurs et en trace qui est compatible avec un dkveloppement dans une zone de subduction associCe h un arc magmatique. L'ornniprCsence de xCnolites et les zones migmatitiques aux contacts des plutons - roches encaissantes indiquent le r61e important de l'assimilation des roches crustales dans le processus magmatique. Les lges de cristallisation U-Pb sur zircon, fournis par les roches ignks des Domaine c6tier mkridional et Domaine insulaire, revklent que durant l'intervalle Jurassique moyen h CrCtacC tardif, il y a eu un glissement gknCral de l'axe magmatique vers l'est. Malgrk l'absence de vieux zircons hCritCs dans la majorit6 des roches, nkanmoins, ils ont CtC dkel6s en traces dans plusieurs plutons du Domaine c6tier mkridional datCs du Jurassique moyen - Crktack sup6rieur. Le caractkre primitif et la variation faible parmi les donnCes des isotopes Nd-Sr des roches ignks du Domaine c6tier meridional datCes du Jurassique moyen i Quaternaire (E,, = +2,4 ? +8,0; i Sr, = 0,7030-0,7042) indiquent que ces roches denvent d'un arc magmatique isotopiquement juvknile. La distribution des valeurs isotopiques le long du dispositif mantellique et la forte variation des valeurs de f,,,,, suggkrent que le magma est issu du manteau appauvri d'un coin mantellique, peu ou aucunement contamink par du vieux matkriel continental isotopiquement CvoluC. Quoique les observations sur le terrain suggkrent que l'assimilation d'une croiite juvCnile ait CtC un processus important durant l'ascension du magma, les donntes isotopiques et gkochirniques ne permettent pas nCanmoins, de faire la distinction entre la derivation des magmas directe du manteau, suivie par un fractionnement et une assimilation crustale, d'avec la fusion massive d'un arc mafique dkrivke d'une croiite plus profonde. [Traduit par la rCdaction] Received March 3, 1994. Accepted March 13, 1995. R.M. Friedman: J.B. Mahoney? and Y. Cui. Department of Geological Sciences, The .University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 124, Canada. Lithoprobe Publication 638. Corresponding author (e-mail: rfriedma@geology .ubc.ca). Present address: Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, WI 54702-4004, U.S.A. Can. J. Earth Sci. 32: 1681-1698 (1995). Printed in Canada / Imprim6 au Canada

Can. J. Earth Sci. Vol. 32, 1995

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/31/15 For personal use only.

Introduction Rocks of the Coast Belt of British Columbia record more than 100 Ma of magmatism along a convergent plate boundary. They make up a segment of the Mesozoic -Cenozoic arc that is exposed along the western margin of North and South America. In Canada and Alaska this arc was built entirely on accreted terranes whose pre-early Tertiary position with respect to the North American craton is uncertain. In southwestern British Columbia the arc is represented by the southern Coast Plutonic Complex and by associated volcanic rocks. In this paper we describe the evolution of MesozoicCenozoic magmatism in southwestern British Columbia, based primarily on Nd- Sr isotopic and geochronologic data for the southern Coast Plutonic Complex. We present new Nd - Sr data for 25 Middle Jurassic to Miocene plutons and 3 associated volcanic rocks, and review U - Pb geochronological, isotopic, and geochemical data for igneous rocks of the southern Coast Belt. These data are used to constrain the nature and history of magmatism in the southwestern Canadian Cordillera. The characterization of Mesozoic- Cenozoic magmatism in southwestern British Columbia provides a framework within which to address questions related to crustal growth and the tectonic evolution of this area and permits comparisons with other segments of the coastal batholith along strike to the north and south. In addition, data from this study may provide insights into the nature and evolution of subductionrelated magmatic systems.

Regional geologic setting of the Coast Plutonic Complex The Coast Belt, one of five morphogeologic subdivisions of the Canadian Cordillera, encompasses parts of coastal British Columbia and southeastern Alaska, and extends northward into southwestern Yukon Territory. It is flanked on the east by the Intermontane Belt and on the west by the Insular Belt (Fig 1). The Coast Plutonic Complex, which underlies most of the Coast Belt, is the largest single concentration of plutonic rock on the western North American margin. It is a long, narrow plutonic belt, about 1700 km long and 50175 krn wide, composed of discrete to coalescing plutons, migmatite, gneiss, and lesser variably metamorphosed volcanic and sedimentary rocks. Its boundaries are defined by the limit of dominantly plutonic rocks (Woodsworth et al. 1991) (Fig. 1). The Coast Plutonic Complex has a northwesterly regional trend defined both by major structures and elongation of plutons. At least 75% of the Coast Plutonic Complex is underlain by rocks of granitic origin and greater than 75% of these are quartz diorites, tonalites, and diorites (Woodsworth et al. 1991). Consequently, it is the least felsic circum-Pacific batholithic belt, having a composition similar to that of average continental crust (Roddick 1983; Woodsworth et al. 1991). Major element, trace element, and rare earth element (REE) geochemical data for southern Coast Plutonic Complex plutons and associated volcanic sequences are consistent with their formation within a subductionrelated magmatic arc (Mahoney 1994; Cui and Russell 1995~). Coast Plutonic Complex plutons range in age from Silurian to Miocene, although dating studies indicate that at least

95% of the bodies are Jurassic to Eocene in age (Woodsworth et al. 1991; van der Heyden 1989, 1992; Friedman and Armstrong 1995). Coeval, cogenetic Mesozoic and Cenozoic volcanic sequences are exposed along the flanks of the plutonic belt and locally form pendants and screens. Regional U-Pb dating studies in the central and southern Coast Plutonic Complex indicate that the axis of magmatism migrated eastward across the Coast Belt during Cretaceous time (van der Heyden 1989, 1992; Friedman and Armstrong 1995). In general, K- Ar biotite and hornblende ages also young towards the east. Such ages are commonly several million years younger than U-Pb ages for the same body (van der Heyden 1989, 1992). Plutons of the Coast Plutonic Complex intrude Cordilleran accreted terranes (Fig. 1). North of about 54"N they intrude pre-Mesozoic basement rocks of the Alexander, Taku and Nisling terranes. Rocks with Alexander and Nisling terrane affinities are thought to extend southward to about 52"N, where they occur as pendants within the core of the complex (Wheeler and McFeely 1991; Boghossian et al. 1993). The eastern flank of the Coast Plutonic Complex intrudes Triassic and Jurassic arc volcanic and sedimentary rocks of Stikinia north of about 51°N, whereas to the south, small early Mesozoic arc (Cadwallader and Harrison) and oceanic (Bridge River) terranes are exposed along the eastern margin of the complex (Fig. 1). The western margin of the southern Coast Plutonic Complex intrudes Wrangellian strata, and pendants and screens of recognizable Wrangellian strata are exposed along the western margin of the southern Coast Belt (Roddick and Woodsworth 1979; Monger 1990). The Coast Plutonic Complex is cut by prominent Late Cretaceous to Tertiary regional fault systems that extend from southeast Alaska through the southern Coast Belt into northern Washington (Tipper 1969; Rubin et al. 1990; Rusmore and Woodsworth 1991; van der Heyden et al. 1994). These fault systems consist primarily of Late Cretaceous west- and east-directed contractional structures, and are coincident with younger Tertiary transcurrent faults.

Southern Coast Belt geology The southern Coast Belt (SCB) for this study is defined as the region between 51" and 49"N, bounded on the west by the Strait of Georgia and on the east by the Pasayten fault (Wheeler and McFeely 1991) (Fig. 1). The western portion of the SCB is dominated by granitoids of the Coast Plutonic Complex, whereas Mesozoic volcanic and sedimentary rocks underlie the majority of the easternmost Coast Belt (Fig. 2). The northwest-trending, west-directed early Late Cretaceous Coast Belt thrust system divides the SCB into three tectonically and lithologically distinct domains (Journeay and Csontos 1989; Journeay and Friedman 1993) (Fig. 2). The western Coast Belt domain extends eastward from the Strait of Georgia to the imbricate zone of the Coast Belt thrust system and is underlain primarily by Middle Jurassic to Upper Cretaceous plutonic rocks and lesser Jurassic to Lower Cretaceous volcanic-arc assemblages. Plutonic rocks of the western domain are flanked on the west by volcanicarc rocks of the Lower to Middle Jurassic Bonanza Group and associated Wrangellian strata, and are flanked on the east by Lower to Middle Jurassic volcanic rocks and associated

Friedrnan et al.

Fig. 1. Terrane map of central and western British Columbia, southwestern Yukon, and southeast Alaska, modified from Wheeler and McFeely (1991). The present study area in the


southern Coast Belt is outlined. Inset map of British Columbia shows the morphogeological belts of the Canadian Cordillera.

ACCRETED TERRANES Inlermodnnc Superferrone

0

100

XW

3Ml km

strata of the Harrison terrane. Several workers have proposed that the Lower to Middle Jurassic volcanic-arc assemblages of the Wrangellia and Harrison terranes were generated in a single magmatic arc and thus provide a tie between terranes of the Insular Belt and those to the east (Roddick 1965; Friedman et al. 1990; Mahoney 1994; Friedman and Armstrong 1995). The close temporal and spatial relationship between Middle Jurassic plutons of the Island Intrusions on Wrangellia and plutons of the SCB strengthens this proposal (DeBari et al. 1995).

Metamorphic grade in the western Coast Belt is generally low, commonly not exceeding greenschist grade. Contacts between granitoids and pre-granitic rocks are variable, ranging from migmatitic zones, most commonly associated with dioritic complexes, to sharp faults (Roddick 1965; Woodsworth et al. 1991). Rocks of the western Coast Belt do not contain regionally developed deformation fabrics but are penetratively deformed in some faults zones and within granitic gneiss complexes (Journeay and Friedman 1993). The central Coast Belt domain is synonomous with the

1684


I


Fig. 2. Geological compilation map of the Coast Belt, 49-51°N, west of the Fraser fault system, including Texada Island of the Insular Belt. Inset map shows the western, central and eastern Coast Belt domains (Journeay and Csontos 1989). Sources are Roddick (1965), Roddick and Hutchison (1973), Roddick and Woodsworth (1977, 1979), Woodsworth (1977), Journeay and Csontos (1989), Monger (1989a, 1990), Journeay (1990), Monger and Journeay (1992), and Mahoney and Journeay (1993). AC, Ashlu Creek pluton; ACF, Ashlu Creek fault; AS, Ascent Creek intrusion; B, Bendor pluton; BF, Bralorne fault zone; BJ, Big Julie pluton; BSZ, Britannia shear zone; C, Cloudburst pluton; CB, Cypress Bowl intrusion; CCBD, Central Coast Belt Detachment fault; CH, Chilliwack batholith; CL, Clendenning pluton; CT, Castle Towers pluton; DP, Dickson Peak pluton; ES, East Sechelt pluton; FC, Furry Creek pluton; GL, Goat Lake pluton; H, Hope; HB, Horseshoe Bay intrusion; HL, Harrison Lake; HLF, Harrison Lake fault; HU, Hurley River pluton; KF, Kwoiek Creek fault; L, Lillooet River intrusion; M, Malibu diorite; MA, Malispina pluton; MC, Mount Clarke pluton; MJ, Mount Jasper pluton; MM, Mount Mason pluton; MR, Mount Rohr pluton; P, Pemberton; PDC, Pemberton Dioritic Complex; PI, Pitt Lake pluton; PL, PrincessEouisa granodiorite; PR, Princess Royal Reach granodiorite; PWF, Prince of Wales fault; Q, Quatam pluton; QB, Quarry Bay intrusion; R, West Redonda diorite; RC, Rogers Creek pluton; S, Squamish pluton; SC, Spetch Creek pluton; SCUZ, Scuzzy pluton; SE, Sechelt pluton; SL, Sackinaw Lake pluton; SM, Sumas Mountain intrusion; SP, Spuzzum pluton; T, Thornbrough intrusion; TI, Texada Island; TL, Thomas Lake pluton; TLF, Thomas Lake fault. Geological contact Faults: Thrust-Reverse

A

0

C ~ C e n o z o i sediments c

Cv:Cenozoic volcanics

Coast Plutonic Complex Tg:Tertiary plutons

Western Coast Belt KG:Gambier-Fire Lake Gp.

Central Coast Belt m ~ ~ s : ~ e t t l e r - ~Ck, h iSchist s m

J1IBH:Bowen-HarrisonLk. Gp.

TrC:Cadwallader

MKg:Mid-Cretaceousplutons

MTi:Twin Island Schkt

TrHe: Hornet Ck. Gneiss

EKg:Early Cretaceousplutons

MSI:Slollicum Schist TrK:Kannutsen Fm. & equiv. & Quatsino Fm., E. Insular Belt Trs: Unnamed sediments

PMC:Cogburn Group

(LKg:Lure

Cretaceous plutons

0

a

LJg:Middle-Late Jurassic plutons gu:Plutons of uncertain age

l?zi

gnu: Undivided gneiss & metamorphic rocks

Eastern Coast Belt JKs:Jura-Cretaceous clastics TrC:Cadwallader Group MJB:Bridge River Group


Friedman et al.

imbricate zone of the Coast Belt thrust system, a westdirected early Late Cretaceous stack of thrust sheets extending from the Central Coast Belt Detachment on the west to the Kwoiek Creek fault in the east (Journeay and Friedman 1993) (Fig. 2). Central Coast Belt rocks are commonly of middle to upper amphibolite metamorphic grade and are penetratively deformed (Journeay and Csontos 1989). Lithologic assemblages within the imbicate zone have been correlated with lower grade rocks in both the Harrison terrane of the western Coast Belt and the Cadwallader and Bridge River terranes of the eastern Coast Belt (Journeay and Friedman 1993; Journeay and Mahoney 1994). Correlations with rocks of the Northwest Cascades and the metamorphic core of the North Cascades have also been suggested (Monger 1991). U-Pb dating of Late Cretaceous plutons that are synand postkinematic with respect to structures of the Coast Belt thrust system constrains the timing of deformation to between 91 and 96 Ma (Journeay and Friedman 1993). Frontal thrusts of the Coast Belt thrust system extend at least 100 krn west of the imbricate zone and are interpreted to have been active at about the same time as fault strands to the east. The eastern Coast Belt domain extends from the Kwoek Creek fault to the Pasayten fault and is underlain by several terranes (Fig. 2). In the west, the Cadwallader terrane consists of an Upper Triassic island-arc assemblage, overlain by Upper Triassic to Lower Cretaceous marine and nonrnarine sediments. The Bridge River terrane includes Mississippian to Jurassic ribbon chert and argillite, mafic and ultramafic rocks of the East Liza Complex, including the Permian Bralorne intrusions, and the Lower Jurassic - Lower Cretaceous Cayoosh Assemblage. The Cayoosh Assemblage is interpreted to depositionally overlie older rocks of this terrane and is correlated with Middle Jurassic to Lower Cretaceous sedimentary and volcanic rocks of the Methow basin to the east (Mahoney and Journeay 1993). Journeay and Mahoney (1994) interpret the Cayoosh Assemblage as an important overlap sequence that links together terranes of the eastern Coast Belt. Distinctive Upper Jurassic to Lower Cretaceous conglomerate that contains granitic clasts of Late Jurassic age overlies diverse basements throughout the SCB. These rocks indicate rapid latest Jurassic uplift and suggest mutual proximity of all terranes west of the Pasayten fault by this time. About 80 - 140 km of dextral strike - slip offset occurred along the Fraser - Straight Creek fault system located near the eastern margin of the SCB during Eocene time (Monger 1989a, 1989b; Coleman and Parrish 1991). Restoration of this displacement places rocks of the eastern and central Coast Belt against those of the metamorphic core of the North Cascades (Fig. 3). Other regionally important Tertiary structures in the SCB include a set of northeast-trending, steeply dipping faults dated as Miocene in age (Fig. 2). A fission-track study of the Coast Belt suggests that the SCB underwent 0- 3.5 km of post-40 Ma uplift, increasing from southwest to northeast, significantly less than uplift documented for the northern Coast Belt (Parrish 1983). Late Cenozoic igneous rocks occur within all domains of the SCB, and include Miocene volcanic and intrusive rocks of the Pemberton belt (Berman and Armstrong 1980; Coish and Journeay 1992), and volcanic flows of the Quaternary Garibaldi belt (Green 1981) (Fig. 2). The Oligocene-

Fig. 3. (a) Sketch map showing present distribution of assemblages along the Fraser - Straight Creek fault system. The Oligicene Chilliwack batholith (CH) cuts the Fraser Straight Creek fault system. (b) Pre-Eocene distribution of assemblages about the future trace of the Fraser - Straight Creek fault system (broken line) restoring 90 km of dextral strike slip along this structure (modified from Monger and Journeay 1992). Stippled area shows the region where inheritance has been documented. CMC, Cascade Metamorphic Core; CK, Chilliwack Group; HaF, Harrison fault; HoF, Hozameen fault; JK, Jurassic and Cretaceous Coast Plutonic Complex granitoids; PF, Pasayten fault; YF, Yalakom fault. Other abbreviations as in Fig. 1.

Pliocene Chilliwack batholith spans much of the age range for both of these suites (Tepper et al. 1993).

Lithologic and geochemical nature of granitoids Plutonic rocks make up at least 75% of the SCB, and are strongly dominated by quartz diorite, tonalite, and diorite, with lesser granodiorite and quartz monzodiorite, and rare gabbroic and felsic rocks (Roddick 1965, 1983). The only discernable compositionalzonation within the southern Coast Plutonic Complex relates to a concentration of dioritic rocks along its western flank. Dioritic complexes are the most lithologically heterogeneous granitoids, and there is a marked increase in textural and lithologic homogeneity in less mafic rocks. Chemically, plutonic rocks of the SCB have an intermediate to mafic character, with an average compositon of quartz diorite (Roddick 1983; Woodsworth et al. 1 9 9 1 ~ ) . The overall chemical compositon approaches that of average continental crust, although Na20 and A1203 contents are distinctly higher and K20 content lower (Roddick 1983). Crustal assimilation was an important process during the emplacement of SCB plutons. The presence of partially digested country rock adjacent to plutons and the occurrence of ubiquitous xenoliths within most plutons provide direct evidence of crustal assimilation. Migmatitic zones are locally developed along contacts between dioritic complexes and roof pendants. Within and adjacent to these migmatitic zones



country rock appears to have undergone various degrees of digestion and in extreme cases has the appearance of inclusion swarms. Inclusions commonly consist of hornblendeandesine granulite and amphibolite, with lesser gneiss, schist, and bedded rocks; granitic inclusions are very rare. The vast majority of inclusions are rounded or subrounded, and are in general more abundant and of larger average size in hornblende diorites and quartz diorites (approx. 5 vol. % overall, locally 30-40 vol. %) than in more felsic intrusions (1 2 vol. %) (Roddick 1965). Plutonic rocks in the SCB have been subdivided into petrologic series based on the presence or absence of modal K-feldspar (Cui and Russell 1995~).Similar classification systems have been used for the Island Intrusions - Westcoast Complex of Vancouver Island (DeBari et al. 1995) and the Patagonia Batholith in Chile (Weaver et al. 1990). The geochemical differences between these series may reflect important differences in their petrogeneses. Geochemical studies by Michael and Russell (1989), Tepper et al. (1993), Mahoney (1994), and Cui and Russell (1995~)provide some of the first whole-rock chemical analyses for plutonic rocks of the SCB. The vast majority of the plutons have mineralogical and geochemical characteristics typical of the I-type plutons as defined by Pitcher (1983) (Woodsworth et al. 1991). They have subalkaline and calcalkaline major element affinities, and are volcanic-arc granites in the classification scheme of Pearce et al. (1984). High large ion lithophile element to high field strength element (LILEIHFSE) ratios and light rare earth element (LREE) enrichments documented for these rocks (Cui and Russell 1995a) suggest that they were generated in a subductionrelated magmatic arc (Hawkesworth et al. 1993). Middle Jurassic plutons have similar trace element and REE trends to overlying, comagmatic Jurassic volcanics of the Harrison Formation and Bowen Island Group (Mahoney 1994; Mahoney et al. 1995). A detailed geochemical study of the Oligocene-Pliocene Chilliwack batholith of the eastern Coast Belt and Northwest Cascades concluded that melting of lower crust was the dominant process controlling its generation (Tepper et al. 1993). The present geochemical database for other SCB plutons supports this model, or one involving direct derivation of magmas from the mantle, followed by fractionation and crustal assimilation.

Geochronology Crystallization ages and U-Pb zircon systematics of more than 40 southern Coast Plutonic Complex plutons help to define the time - space pattern of rnagmatism within this batholithic belt (Friedman and Armstrong 1995). Figures 4a 4e show the distribution of Middle Jurassic and younger plutonic suites of the southern Coast Plutonic Complex as a function of age and provide a framework within which to discuss the time-space patterns of plutonism in the region. Middle- Upper Jurassic (176-145 Ma) plutons comprise the oldest granitoids of the southern Coast Plutonic Complex. ~ h e s erocks are exposed in the western Coast Belt domain within western and eastern outcrop belts separated by Cretaceous plutons (Fig. 4a). The eastern outcrop belt is characterized by large tracts of coalescing plutons ( ~ i 2). ~ . The Middle Jurassic Mount Jasper pluton is coeval and

comagmatic with hypabyssal and extrusive rocks of the Harrison terrane and defines the eastern limit of the eastern outcrop belt (Mahoney et al. 1995) (Fig. 2). The western belt is adjacent to plutons of the Lower and Middle Jurassic Island Intrusions west of the Strait of Georgia. We interpret the Island Intrusions to have been generated in the same magmatic system as plutons of the Coast Plutonic Complex, and suggest the Island Intrusions represent an outlier of Coast Plutonic Complex granitoids separated from the main body of the complex by a thin veneer of Wrangellian crust. This interpretation suggests that the Island Intrusions represent the earliest phase of southern Coast Plutonic Complex magmatism, and that the age distribution of Middle Jurassic plutons within the Island Intrusions and SCB simply represent a Middle Jurassic widening of the magmatic arc ahon one^ 1994; Friedman and Armstrong 1995; Debari et al. 1995). The Middle Jurassic widening was followed by an eastward shift in the western limit of magmatism, which led to the cessation of Island Intrusion - Bonanza Group igneous activity by Late Jurassic time (Friedman and Armstrong 1995; Mahoney 1994). Middle Jurassic plutons provide an unequivocal tie between Wrangellia and the Harrison terrane. Correlation of the Early -Middle Jurassic volcanic-arc assemblages of the Bonanza Group, Bowen Island Group, and Harrison Lake Formation suggest an even earlier tie between Wrangellia and the Harrison terrane (Roddick 1965; Friedman et al. 1990; Mahoney 1994). Lower Cretaceous (145 - 112 Ma) plutons are exposed along the western and eastern sides of the western Coast Belt, but &e concentrated in the west near the Strait of Georgia where they primarily intrude Middle -Upper Jurassic plutons and represent youngest known plutons in the area (Figs. 2, 36). Other than a small Lower Cretaceous stock along the eastern shore of Vancouver Island, Mesozoic plutons of postMiddle Jurassic age are unknown west of the Strait of Georgia. Coeval volcanic rocks from the lower and middle portion of the Lower Cretaceous Gambier Group occur as pendants in the western and central part of the western Coast Belt. The distribution of Middle Jurassic to Lower Cretaceous plutons in the Insular and southern Coast belts indicates an eastward shift in the western limit of magmatism across Vancouver Island to the Strait of Georgia during this time frame. A portion of the Pemberton Diorite Complex lies along the eastern margin of the western Coast Belt domain and defines the known eastern limit of magmatism for this time interval (Figs. 2, 3b). Mid-Cretaceous (late Early to early Late Cretaceous, 112-90 Ma) plutons occur in both the western and central Coast Belt domains, and occur within the Coast Belt thrust system as syn- and postdeformational intrusions (Figs. 2, 3c). U -Pb dating of syn- and postkinematic mid-Cretaceous intrusions bracket the timing of deformation along the Coast Belt thrust system to between 91 and 96 Ma (Journeay and Friedman 1993). Mid-Cretaceous rocks intrude older plutons and volcanic sequences in the western Coast Belt, and intrude sedimentary and volcanic sequences on a number of small terranes in the central Coast Belt (Fig. 2). The Spetch Creek and Spuzzum plutons (ca. 100- 105 Ma) are the oldest plutons in the central Coast Belt (Fig. 2). The areal distribution of mid-cretaceous plutons indicates a 30 krn northeastward migration of the axis of magmatism relative to Early Creta-

Friedman et al.

Fig. 4. Distribution of plutonic rocks of known age in the southern Coast Belt modified from Friedman and Armstrong


(1995). Shorelines, faults, and the boundaries of the western, central, and eastern Coast Belt domains (broken lines) are plotted for reference. Plutons are stippled. (a) Middle -Late Jurassic (176 - 145 Ma). (b) Early Cretaceous (145 - 112 Ma). (c) Mid-Cretaceous (1 12-90 Ma). (d) Late Cretaceous (90 -65 Ma). (e) Tertiary -Quaternary (65-0 Ma).

ceous activity (Fig. 3c). The presence of mid-Cretaceous plutons in the metamorphic core of the North Cascades suggests that this belt continues to the south (Tabor et al. 1987) (Fig. 2). Latest Cretaceous to Tertiary (90- 65 Ma) plutons occur in all three tectonic domains but are concentrated along the

eastern flank of the SCB where they define the most easterly excursion of Mesozoic plutonism in this area (Fig. 3d). They primarily intrude rocks of the Cadwallader and Bridge River terranes and the overlying Cayoosh Assemblage. The western limit of magmatism migrated nearly 100 krn to the northeast between about 94 and 87 Ma (Friedman and Armstrong



1995). This eastward shift overlaps in time with contraction in the Coast Belt thrust system and could be the result of westward translation of upper plate rocks relative to the zone of magma generation, or a shift in the locus of magmatism resulting from a shallowing of the subducting slab (Friedman and Armstrong 1995). Tertiary to Quaternary (65- 0 Ma) plutons and associated volcanic rocks occur in all three tectonic domains and, like Late Cretaceous plutons, are concentrated along the eastern flank of the SCB. Eocene plutons are sparse, and are limited to the eastern Coast Belt and areas to the east. Oligocene to Pliocene plutons and associated volcanic rocks form a northwest-trending belt that extends from the Fraser - Straight Creek fault system in northern Washington into the western Coast Belt. This suite includes the Chilliwack and Mount Barr batholiths and the Rodgers Creek and Salal Creek plutons, and is interpreted to be related to the Pemberton volcanic belt (Souther 1991; Woodsworth et al. 1991). The Pemberton volcanic belt is parallel to and older than the Late Tertiary to Quaternary Garibaldi Volcanic Belt (Souther 1991). Both the Pemberton and Garibaldi volcanic belts are interpreted to be the products of renewed subduction resulting from interaction between North America and the subducting Farallon plate (Souther 1991). There is a substantial magmatic gap (approximately 12 Ma) between the emplacement of Eocene plutons in the eastern Coast Belt and the initiation of magmatism in the Oligiocene to Pliocene Pemberton volcanic arc. The presence of this magmatic gap, coupled with the distinct spatial reorganization represented by the Pemberton and Garibaldi volcanic belts, suggests that Late Tertiary to Quaternary igneous activity represents a younger magmatic event distinct from the Jurassic to Early Tertiary magmatism that formed the vast majority of the southern Coast Plutonic Complex.

Southern Coast Belt zircon systematics Geochronologic studies have documented generally uncomplicated U-Pb systematics for zircon from SCB igneous rocks (Friedman et al. 1990; Parrish and Monger 1992; Parrish 1992; Friedman et al. 1992; Friedman and Armstrong 1995). Zircons commonly yield concordant dates or have undergone minor Pb loss. However, some SCB igneous rocks contain evidence of pre-Mesozoic inheritance. Igneous rocks with inheritance primarily occur in an area that extends from west of Harrison Lake to the Fraser fault, and intrude or overlie the Harrison terrane or terranes of the eastern and central Coast Belt. (Fig. 3a). Due to the very small component of inherited zircon commonly occurring in these rocks, the age(s) of inheritance is poorly constrained. Age estimates for inherited components provided below are based on upper intercepts of regression lines fit to U - Pb data, with 20 errors (Friedman and Armstrong 1995). The only SCB pluton west of the Harrison Lake area that displays demonstrable inheritance is the Lower Cretaceous Goat Lake pluton, which lies in the northwestern part of the study area south of Toba Inlet (Fig. 2). An estimate for the average age of inheritance in this pluton is 0.87';:fi Ga. Given the quoted precision, possible sources of this inheritance are numerous. However, some reasonable possibilities include the middle-upper Paleozoic Sicker Group of Wran-

gellia, or the older Nisling terrane, which underlies part of the northern Coast Belt and occurs in pendants about 300 km north of this pluton. Lower to Middle Jurassic plutons of the Island Intrusions on Wrangellia display inheritance of probable late Paleozoic age, most likely derived from local Sicker Group country rocks (Parrish and McNicoll 1992; DeBari et al. 1995). A number of Middle Jurassic to Upper Cretaceous igneous rocks in the Harrison Lake area provide evidence of Paleozoic and Precambrian inheritance (Fig. 3a). The Middle Jurassic Sumas Mountain intrusion, which is exposed southeast of Harrison Lake, has an inherited Pb component with an upper intercept age of 0.44?!:;! Ga. The middle -upper Paleozoic Chilliwack Group, which is exposed nearby, could be the source of this inheritance. Two Middle Jurassic rocks (ca. 166 Ma; Mahoney et al. 1995), the high-level Hemlock Valley stock and comagmatic rhyolite in the Echo Island member of the Harrison Lake Formation, give upper intercepts of 2.7::;: Ga and 1.3:;:;; Ga, respectively. It is notable that the deeper level equivalent of these rocks, the nearby Mt. Jasper pluton, shows no trace of inheritance (Friedman and Armstrong 1995) (Fig. 2). Several Upper Cretaceous rocks in the Harrison Lake area contain Precambrian inheritance, including a felsic volcanic flow from the Brokenback Hill Formation, a sample of Breakenridge granitic gneiss, and the Scuzzy pluton (Parrish and Monger 1992) (Fig. 2). The Scuzzy pluton intrudes metasediments of the Settler schist, a metasedimentary unit known to contain detrital zircons that are Precambrian or contain Precambrian inheritance (Gabites 1985). In addition, both the Upper Cretaceous Mount Rohr and Hurley River plutons may display minor inheritance, although the ages of inheritance are difficult to confidently evaluate (Friedman and Armstrong 1995). Inheritance in SCB rocks has been documented as far east as the Fraser - Straight Creek fault system (Figs. 2, 3). No pre-Mesozoic inheritance has been recognized in Jurassic and Cretaceous rocks of the Eagle Plutonic Complex east of the Fraser - Straight Creek fault system (Greig et al. 1992). However, prior to Eocene dextral offset of 80-140 km along the Fraser - Straight Creek fault system, southeastern SCB rocks lay west of the metamorphic core of the North Cascades (Fig. 3b), where Precambrian rocks have been documented in the Swakane terrane (Tabor et al. 1987). Precambrian rocks are also known to exist in the western Cascades, where the Yellow Aster complex is exposed in small fault-bounded slivers (Mattinson 1972). Rocks lithologically correlated with the Yellow Aster complex occur in the southeastern Coast Belt, but none have yet been reliably dated (Monger 1989a). It is reasonable to suggest that Precambrian units of the metamorphic core of the North Cascades and Northwest Cascades may have been the source of inheritance in rocks in the southeastern Coast Belt. An alternative explanation for Precambrian inheritance is that it was derived from detrital zircons in relatively young (early Mesozoic?) sedimentary rocks. Detrital zircons in the Settler schist of the central and eastern Coast Belt that are of Precambrian age or contain Precambrian inheritance lend some credence to this possibility (Gabites 1985). A singlegrain detrital zircon U -Pb geochronology study of early Mesozoic sedimentary and metasedimentary rocks of the

Friedman et al.

1689


Fig. 5. Map of study area showing sample locations and isotopic data. Closed symbols, data from this study; open symbols, data from Cui and Russell (1995~).Identification number (ID) for each sample locality is given in Tables 1 and 2. Average 20 errors for data from this study are shown.

Coast Plutonic Complex plutons -Tg:Tertinry LKg:Late Cretaceous plutom 1Kg:Mrd-Cretaceous plutom EKg:Early Cretaceous plutom LJg:Middle-Lote Jurassic plutom gu:Plutons of uncertain age

Western Coast Belt KG:Gambrer-Frre Lake Gp. J1H:Bowen-Harnson U.Gp

...... MTi:j"ww I&nd

Schist

MSI:SloUrcum Schrst TrK:Kamutsen Fm & equrv. & Quatsino Fm , E. Insular Belr Trs:Unnamed sedrments

rn

gnu:Undrvrded gnerss & metamotphrc rocks

Central Coast Belt m 1 ~ s : ~ e t t l e r - ~ h iCk. s r Schist n TrC:Cadwallader Group

I : , TrHe: Hornet Ck. Gneiss P1C:Cogbum Group

Eastern Coast Belt JKs:Jura-Cretaceous clastrcs TrC:Cadwallader Group MJB:Brrdge River Group

eastern and central Coast Belt would help to confirm or refute this possibility.

-

Nd Sr isotopic systematics The Nd-Sr isotopic systematics of southern Coast Plutonic Complex plutons were investigated to provide constraints on their magmatic origins, probe the isotopic character of the

underlying crust, and determine the level of interaction between arc magmas and the preexisting crust. The sampling programme (n = 26) was designed to encompass the entire age and compositional range of SCB plutons and to provide complete regional coverage (Figs. 2, 5; Table 1). Volcanic rocks from the Lower to Middle Jurassic Bowen Island Group and Lower Cretaceous Gambier Group were analyzed to evaluate the isotopic composition of coeval, probable

1690



Table 1. Sample rock type, location, and age. Sample

Unit

Rock type

JR148-88 MV88-37 RF89-2 RF88-MD MV89-565 RF88-THB RF89-8 MV89-507 RF89-18 RF88-PLG RF89-13 RLA-CB RF88-SL RF89-PDC MV88-29 RF89-SC RF88-PRR SPUZZ JR268-88 RF89-17 MV89-644 RF89-HRC EHB164-289 WV84-BEND PJM-B1 PJM-B8 JR88-16 MV89-638 THSA TH6A 9022A 9023 9006 901 1 9012 9013 9014 9015 9016 9020 9001 9032 9004 9030 9033 9003 9009

Port Douglas Mount Jasper Sumas Mountain Malibu Carlson Lake Thornbrough Ryan River Ashlu Creek Goat Lake Princess Louisa Quatam Cypress Bowl Sackinaw Lake Pemberton Complex Mount Clarke Spetch Creek Princess Royal Spuzzum Ascent Creek Big Julie Castle Towers Hurley River Scuzzy Bendor Bendor Bendor Rogers Creek Bowen Island Group Gambier Group Gambier Group Soo River Soo River Cloudburst Cloudburst Cloudburst Ashlu Creek Ashlu Creek Ashlu Creek Ashlu Creek Callaghan Creek Cypress Bowl Spetch Creek Squamish Cadwallader Group Cadwallader Group Gambier Group Garibaldi Group

Bt granodiorite Bt -Hbl tonalite Hbl granodiorite Hbl diorite Hbl- Bt Qtz diorite Hbl diorite Bt tonalite Hbl diorite Hbl diorite Bt tonalite Hbl diorite Hbl tonalite Hbl -Bt tonalite Hbl diorite Hbl tonalite Hbl Qtz diorite Hbl -Bt granodiorite Bt -Hbl diorite Tonalite Bt granodiorite Hbl Qtz monzonite Hbl -Bt qtz diorite Bt Qtz monzonite Bt tonalite Bt tonalite Bt tonalite Hb granodiorite Felsic metavolcanic Basalt Dacite Qtz diorite Qtz diorite Aplite Qtz diorite Mafic dike Tonalite Qtz diorite Tonalite Hbl diorite Hbl diorite Granodiorite Tonalite Leucogranite Andesitic tuff Andesite Tuff Tuff

Age (Ma)b

110:: 103:: 103+2 ca. 100d 96+6' 94k2 91 +3' 87+2 84f 1 64';' 64';' 64';' 16+le 185';f 114f 808 114 808 Late Jurassic Late Jurassic 147f O.Sh 147+0.Sh < 147+0.5 145:;' 145';' 145Tp 145'i2 128+8' 125': 103': 101+2j Late Triassic Late Triassic Early Cretaceous Late Tertiary Quaternary

+

Lat. (N)

Easting 556876 563755 562751 439894 437620 467391 494084 466777 412740 443728 375482 483900 430332 513414 569085 521797 439147 606737 554509 413188 505742 501295 591475 527159 521732 525400 538828 459305 493621 493621 508679 508679 490768 488979 488979 483713 477383 477383 477383 491658 482715 521664 489541 520032 525436 482715 490768

"Identification numbers used in Fig. 5; 31-47 are from Cui and Russell (19956). bAll dates are U-Pb zircon from Friedman and Armstrong (1995) unless otherwise noted. 'Journeay and Friedman 1993; U-Pb zircon. dGabites 1985; U-Pb zircon. 'R.M. Friedman, unpublished data. fFriedman et al. 1990; U-Pb zircon. gHeah 1982. hParrish and Monger 1992; U-Pb zircon. 'Miller 1979; K- Ar hornblende. JR.L. Armstrong and P. Shore, The University of British Columbia Geochronology Laboratory, unpublished data; U-Pb zircon.

Northing

Friedman et al. Fig. 6. Plot of E,, vs. Sri ratio for rocks from the southern Coast Belt. Closed symbols and bold open square, this study; ; other open symbols, data of Cui and Russell ( 1 9 9 5 ~ )plus sign, stratified rocks, with data from this study and Cui and Russell ( 1 9 9 5 ~ )See . legend for age designations. IAV, and IAV,, contaminated and uncontaminated island arc volcanic fields, respectively.

"S-


;\

-,qA

Fig. 7 . (a) Plot of E,, vs. age for Jurassic and younger southern Coast Belt ignwus rocks. (b) Plot of Sr, ratio vs. age for Jurassic and younger southern Coast Belt igneous rocks. Symbols as in Fig. 6. 10

,/

;w*+

-,

\I

*

FY.

"

"I,,

-

Crust

a

comagmatic, extrusive rocks. Two volcanic rock samples of the Late Triassic Cadwallader Group (Cui and Russell 1995a), which make up a portion of the basement in the eastern Coast Belt, were analyzed to provide partial control on the isotopic composition of the country rock.

Results Initial EN,^ values and Sri ratios vary from +2.4 to 8.9 and 0.7030 to 0.7043, respectively, for the entire sample set, although plutonic rocks display slightly more restricted ranges of +2.4 to +8.0 and 0.7030-0.7042 (Figs. 5-8; Table 2). The isotopic homogeneity of the sample set is striking, with approximately 85% of the samples having EN,^ values between +4.0 and +6.5 (Fig. 6; Table 2). The overall isotopic homogeneity of plutonic rocks in the SCB is evident on an EN,^ versus Sri correlation diagram (Fig. 6). The majority of samples, with the exception of some Late Cretaceous plutons and Triassic and Jurassic volcanic rocks, cluster in the upper portion of the uncontaminated volcanic-arc field, slightly overlapping mid-oceanicridge basalt (MORB) values. The data plot along or very close to the mantle array defined by young ocean island basalts (DePaolo and Wasserburg 1977) (Fig. 6). Displacement of samples of volcanic rock from the Upper Triassic Cadwallader Group and Lower to Middle Jurassic Bowen Island Group to Sri values of above the mantle array is interpreted to be the result of contamination by seawater Sr derived from hydrothermally altered subducted oceanic crust. The slightly more evolved isotopic values displayed by some Late Cretaceous plutons indicate that these bodies and

Fig. 8. (a) Plot of E,, vs. distance across the strike of the Coast Belt from southwest to northeast for Jurassic and younger southern Coast Belt ignwus rocks. (b) Plot of Sr, ratio vs. distance across the strike of the Coast Belt from southwest to northeast for Jurassic and younger southern Coast Belt igneous rocks. Symbols as in Fig. 6.

+

0

20

40

60

80

100

120

140

Distance (krn, SW to NE)

160 I S D 200

Spetch Creek Princess Royal Spuzzurn Ascent Creek Big Julie Castle Towers Hurley River Scuzzy Bendor Bendor Bendor Rogers Creek Bowen Island Group Gambier Group Gambier Group

Sackinaw Lake Pembenon Complex Mount Clarke

Cypress Bowl

Quatam

Carlwn Lake Thombmgh Ryan River Ashlu C m k Goat Lake Princess Louisa

Malibu

Port Douglas Mount Jasper Sumas Mountain

Unit

Age (Ma)

Rbb (ppm)

Slb (ppm) 87Rb/86Sr

87Sr/86Sr measured

Notes: Reported errors for isotopic ratios are at 2u of the mean. "Identification numbers used in Fig. 5 . h~eprnducibilityof Rb and Sr concentrations is 5% or 1 ppm, whichever is greater. 'Reprducibility of Sm and Nd concentrations is 2% or better.

IDa

Table 2. Sm-Nd and Rb-Sr data for the southern Coast Belt. Smc 87Sr/86Sr(i) (ppm)

Ndc @pm)

'43Nd/I4"Nd 147Sm/144Nd measured EN,~(o)


hNd(,)

f

m

(Ma)

t ( ~ ~ )

N

w

1

/

,

1 , ,

I

I

,


i

Friedman et al.

(or) their country rocks are isotopically distinct from the majority of rocks in the region. Plutonic rocks in the southern Coast Plutonic Complex display a relatively restricted range of 6Nd and Sri values, indicating rather uniform source characteristics through time. However, small-scale isotopic fluctuations indicate some degree of magmatic heterogeneity. This heterogeneity probably reflects temporal and spatial variations in the isotopic character of assimilated crustal material. Our analysis of fine-scale isotopic fluctuations emphasizes EN,J over Sri, due to the greater mobility, and therefore the lower reliability of Rb and Sr relative to that of Sm and Nd. A plot of EN^ versus age for SCB plutons and associated volcanic rocks suggests that, within the limited range of isotopic values, there is a relative increase in EN^ heterogeneity through time (Fig. 7a). Pre-150 Ma plutons, which extend across the entire width of the western Coast Belt, have a narrow range of EN,J values, from 5 to 6 (n = 7; Figs. 5, 7a). These values overlap with partially coeval LowerMiddle Jurassic arc volcanic rocks of the Harrison Lake Formation (€Nd = +4 to +6) and the slightly more primitive Bowen Island and Bonanza groups (€Nd = +5 to +8) (Mahoney 1994; Mahoney et al. 1995). Upper Jurassic and Lower Cretaceous plutons are more isotopically variable, with a composite range of EN^ of 3.8 to +7.3 (n = 18; Figs. 5, 7a; Table 2). Upper Cretaceous plutons display a distinct shift to more evolved isotopic values relative to older plutonic suites, and exhibit the widest range of EN,^ values (+2.4 to +6.5) in the region. Late Tertiary and younger rocks are volumetrically subordinate to the older suites, but display a similar isotopic range (€Nd = 3.3 to +6.9) (Fig. 7a) (Tepper et al. 1993). There is a distinct temporal fluctuation in isotopic values superimposed on the gradual increase in heterogeneity with time within plutons of the southern Coast Plutonic Complex. A graph of EN^ versus age reveals significant decreases in EN^ values between approximately 125 and 135 Ma and again at approximately 60- 80 Ma (Fig. 7a). These decreases in hNd values are supported by corresponding increases in Sri values during approximately the same time periods (Fig. 76). The periods of lower EN^ values are interspersed between periods of higher EN^ values, giving the appearance of a weak cyclicity in isotopic values from Middle Jurassic to Late Cretaceous time (Figs. 7a, 76). Variations in isotopic composition are most likely due to the isotopic character and amount of assimilated material incorporated into these rocks. Isotopic values are plotted as a function of distance from southwest to northeast across the strike of the arc in Figs. 8a and 86. Pre-Upper Cretaceous rocks display no coherent regional isotopic trends as a function of geographic position across the arc. However, analysis of the spatial distribution of isotopic values within individual magmatic suites suggests that the most primitive plutons in the Early Cretaceous and mid-Cretaceous suites are concentrated along the eastern side of their respective outcrop belts, near the eastern margin of the western Coast Belt (Fig. 5). There is a distinct shift to more evolved EN^ and Sri values at approximately 175 km across the arc (Figs. 2, 5, 8a, 8b). The geographic position of this shift corresponds to the approximate boundary between the central and eastern Coast Belt domains, and the more evolved values are derived solely from the Upper Cretaceous plutonic suite (Figs. 2, 5).

+

Fig. 9. Plot off,,,

vs. E,.

Symbols as in Fig. 6.

m

O'm

+

+

+

Comparison of Nd and Sm geochemical and isotopic signatures provides additional insight into the nature of southern Coast Plutonic Complex magmatism. A comparison of EN* with fSmlNd,an expression of the S d N d ratio of a sample relative to chondritic values (fSmlNd= [(147Sm/144NdCHUR/ 144Sm/144N&uR)-1]; notation of DePaolo and Wasserburg 1976) evaluates the light rare earth element (LREE) enrichment of a sample relative to its isotopic signature. Differences in LREE enrichment between samples results in differing values, resulting in vertical displacements on this diagram (Fig. 9). Increasing LREE enrichment produces decreasing fSmlNd values, and LREE depletion leads to higher values. Igneous rocks of the SCB display a wide range of negative fSmlNdvalues, indicative of variable degrees of LREE enrichment, a feature typical of magmatic arc systems (Fig. 9). Note that the wide range of fSmlNd values contrasts with the limited range of EN^ values, indicating that LREE enrichment in the SCB is not the result of assimilation of old, LREE-enriched (low S d N d ) crust, as there is no corresponding shift to more evolved EN^ values. Instead, the range of fSmlNdvalues documented for SCB igneous rocks probably results largely from magmatic processes such as partial melting of mantle wedge or lower crust and fractional crystallization of magmas, both of which result in increasing LREE enrichment and decreasing 147Sm/144Ndratios (Shirey and Hansen 1986).

fSmlNd

Discussion of isotopic results Igneous rocks of the southern Coast Plutonic Complex display significant isotopic homogeneity among units of all ages and compositions. The restricted range of EN,^ and Sri values within plutons and associated volcanic rocks of Jurassic to Tertiary age in the Coast Plutonic Complex is indicative of an isotopically uniform magmatic source and juvenile assimilated crust. The range of EN^ and Sri values exhibited by rocks of the SCB are typical of uncontaminated island-arc magmas (DePaolo and Wasserburg 1977; DePaolo and Johnson 1979). The alignment of isotopic values along the mantle array, adjacent to MORB values, suggests magma



was derived from a juvenile isotopic reservoir, which precludes magmatic interaction with -appreciable quantities of subducted oceanic sediments or isotopically evolved crustal material (DePaolo and Wasserburg 1977). The most likely isotopic reservoirs include subducted oceanic crust, depleted mantle wedge, or isotopically juvenile crustal material (Depaolo and Johnson 1979). The restricted range of Sri values suggests that the magmatic system did not receive significant proportions of material derived from hydrothermally altered oceanic crust, which would drive Sri values to the right of the mantle array (Fig. 6 ) (DePaolo and Wasserburg 1977). However, analysis of SmINd values and fsmlNd patterns indicates igneous rocks of the SCB are LREE enriched, and are therefore probably derived from a subduction-relatedmagmatic svstememiched by an elemental flux from the subdictin s i b (Shirey and Hansen 1986; McCulloch and Gamble 1991). The absence of displaced Sri values may therefore be indicative of a lack of hydrothermal alteration in the subducting slab (DePaolo and Johnson 1979). The juvenile isotopic character of rocks of the SCB rules out magmatic interaction with significant amounts of old, isotopically evolved crust. The level of magmatic interaction with preexisting juvenile crust is more difficult to assess. Igneous rocks of the SCB intrude or ascended through accreted terranes including Wrangellia, Cadwallader, Bridge River, and Methow terranes (Fig. 2) (Monger and Journeay 1992). These terranes are primarily composed of rocks of oceanic and volcanic-arc affinity, which are dominated by juvenile isotopic values (Samson et al. 1991a, 1991b; Andrew et al. 1991 ; Mahoney 1994; Mahoney et al. 1995). The presence of ubiquitous xenoliths and migmatitic complexes at pendant -pluton contacts indicates assimilation of country rock was an important process during the emplacement of plutons in the SCB. Incorporation of isotopically juvenile terrane material into ascending magmas would only slightly displace isotopic values along the mantle array and would not significantly increase the heterogeneity of a juvenile magmatic system. Analysis of depleted mantle ages (tDM,172-553 Ma; Table 1) indicates such ages are consistently older than crystallization ages throughout the SCB, which supports assimilation of juvenile terrane material into the magmatic system. The possibility of assimilation of small amounts of Precambrian crust cannot be ruled out. as the isotopic character of SCB plutons permits incorporation of up to 9% of continental material, assuming a two-component mixture between MORB and Proterozoic crust (Cui and Russell 1 9 9 5 ~ ) .However, incorporation of even a small amount of old continental material requires an extremely efficient mixing mechanism to explain the regional isotopic homogeneity of the southern Coast Plutonic Complex. Fine-scale temporal fluctuations in EN^ suggest that there is a weak cyclicity in isotopic composition through time evident within plutons of the southern Coast Plutonic Complex (Fig. 7 ) . We interpret this cyclicity to represent variations in the degree of assimilation through time. The degree of assimilation depends upon the rate of emplacement, residence time in the crust, and the geometry of the ascending plume. During periods of high magmatic activity, large volumes of magma are emplaced rapidly and do not substantially interact with country rock, whereas during periods of lower mag-

matic activity, the rate of emplacement is slower, and there is a higher degree of assimilation of country rock. There is little or no correlation between isotovic values and geographic position within the arc for the majority of pre-Upper Cretaceous rocks, although there is an apparent tendency for the most isotopically primitive Early and midCretaceous plutons to be near the eastern limits of their respective outcrop belts. There is a distinct shift to slightly more evolved isotopic values approximately 175 km from the western edge of the Coast Plutonic Complex, in a position that roughly corresponds to the boundary between the central and eastern Coast Belt domains (Figs. 5, 8a, 8b). The isotopic shift in this area is a function of both space and time, as the slightly evolved plutons in this portion of the arc are Late Cretaceous in age. The Late Cretaceous belt of plutons represents a shift in the locus of arc magmatism approximately 100 krn to the east of its former position. This lateral shift in magmatism resulted in pluton emplacement into a more heterogeneous crust than older plutons (Figs. 2, 5). Plutons from this suite display an EN^ range of +2.4 to +6.5, and include the Scuzzy pluton, which yields the lowest isotopic value determined for plutons in the SCB and displays a component of inherited Pb in its zircon systematics. The pluton was sampled approximately 2 km west of the contact with the enveloping Settler schist. The Settler schist has not been analyzed for EN^, but the presence of an old zircon component suggests that there was, at least in part, an old source for these metasediments (Gabites 1985). It is reasonable to assume that assimilation of Settler schist by the Scuzzy pluton could account for the relatively low (+2.4) EN^ value of the unit. In summary, the restricted range of and Sri values exhibited by a representative suite of plutons and associated volcanic rocks in the SCB suggests that magma was derived primarily from a depleted-mantle source in the wedge above a subducting slab. Assimilation of juvenile terrane material was an important process during magma emplacement, but relative contributions from each isotopic reservoir are dificult to quantify. Nd - Sr isotopic systematics indicate that the southern Coast Plutonic Complex consists almost entirely of juvenile, mantle-derived material, but do not allow us to discriminate between direct derivation from the mantle accompanied by fractionation and minor assimilation of juvenile crustal rocks and wholesale partial melting of a lower crust dominated by preexisting accreted terranes composed of juvenile, mantle-derived basaltic arc material (Tepper et al. 1993; Cui and Russell 1 9 9 5 ~ ) . The southern Coast Plutonic Complex is among the most isotopically juvenile plutonic systems along the western margin of North America. The vast majority of other Mesozoic plutonic systems, including the Peninsular, Sierra Nevada, and Idaho batholiths, northern Coast Plutonic Complex, and plutons in southern Quesnellia and the North Cascades, display at least some degree of interaction with old, isotopically evolved continental crust (DePaolo 1981; Farmer and DePaolo 1983; Fleck 1990; Samson et al. 1991a; Ghosh 1995). With the exception of the northern Coast Plutonic Complex, these plutons commonly have isotopically juvenile western edges, and increasingly evolved isotopic values to the east, indicating interaction with continental crust (Barnes et al. 1992). Plutons in the Klamath Mountains, Alaska

1 ~

1

iI

1 '

1

i

,

I 1


Friedman et al.

Range batholith, and the Aleutian arc systems both have isotopic ranges similar to those of the southern Coast Plutonic Complex, indicating little or no interaction with continental crust. The ramifications of the spatial distribution of juvenile versus evolved batholiths along the western North American margin is unclear. It is clear, however, that long-lived batholithic belts, with their attendant high geothermal gradients, large magmatic fluxes, and relatively high degrees of assimilation, make effective probes of the crustal section into which they are emplaced (Perry et al. 1993).

Summary and conclusions The southern Coast Plutonic Complex is a long-lived, isotopically juvenile, batholith belt built across a composite basement of accreted terranes of juvenile arc and oceanic character. Plutons in the SCB are primarily of intermediate composition and contain large volumes of quartz diorite, diorite, and tonalite, lesser granodiorite, and minor felsic and gabbroic bodies. Dioritic masses are lithologically heterogeneous and commonly contain abundant roof pendants separated by migmatitic zones. Pendant material appears to have undergone various degrees of anatexis within these rnigmatite zones. The volume of the ubiquitous xenoliths in SCB plutons indicates that assimilation of crustal material was an important process during magma emplacervent. The vast majority of plutons in the region have mineralogical and geochemical characteristics typical of the I-type plutons, with subalkaline and calc-alkaline major element affinities. High LILEIHFSE ratios and LREE enrichments documented for these rocks suggest that they were generated in a subduction-related magmatic arc (Hawkesworth et al. 1993; McCulluch and Gamble 1991; Cui and Russell 1995~). The initiation of Mesozoic subduction-related arc magmatism in southwestern British Columbia took place during Early Jurassic time, with generation of the Bonanza Volcanics and associated granitoids of the Island Intrusions and Westcoast Complex (Armstrong 1988; Andrew et al. 1991; Mahoney 1994; DeBari et al. 1995). Volcanism rapidly spread eastwards into the SCB, and is represented by, from west to east, the Bonanza, Bowen Island, and Harrison arc sequences. By Middle Jurassic time, coincident with emplacement of the youngest granitoids of the Island Intrusions, the earliest phase of SCB plutonism began. Plutonism extended from ~ r a n ~ e l lon i a the west to ~ a r r i s o nterrane on the east, and Middle Jurassic plutons definitively link Wrangellia and Harrison terranes together by ca. 165 Ma. The western limit of magmatism migrated eastward across the Strait of Georgia during Late Jurassic time. Uplift and erosion were concomitant with plutonism, and Middle to Late Jurassic granitoids of the SCB were uplifted and shed detritus into adjacent basins during Latest Jurassic to Early Cretaceous time. Plutonism continued in the western Coast Belt domain throughout Early Cretaceous time. Magmatism spread into the central Coast Belt at about 100 Ma, and plutons were emplaced into the Cadwallader terrane and into metasediments correlative with those in the North Cascades. The western limit of plutonism moved northeastward across the SCB throughout Early to Late Cretaceous time. Plutonism was coincident with -active west-directed thrusting in the Coast Belt thrust system, and mid-cretaceous plutons are

both syn- and postkinematic with respect to the thrust system (Journeay and Friedman 1993). There was an abrupt shift in the locus of magmatism of almost 100 km to the northeast at approximately 90 Ma, and Upper Cretaceous plutons are restricted to the eastern Coast Belt, where they intrude Cadwallader, Bridge River, and Methow terranes. Inheritance of Precambrian age is uncommon in zircon from SCB igneous rocks, but has been documented for several Middle Jurassic and Upper Cretaceous rocks associated with or adjacent to thi Harrison terrane (Fig. 3). Precambrian units in the metamorphic core of the North Cascades and western Cascades and correlative units in the southeastern Coast Belt may reasonably be the source of inheritance in rocks east of the Harrison fault. Alternatively, Precambrian inheritance may have been derived from detrital zircons in relatively young (early Mesozoic?) sedimentary rocks that were incorporated into the ascending magma. Detrital zircons in the Settler schist of the central and eastern Coast Belt that are Precambrian or contain Precambrian inheritance support this hypothesis (Gabites 1985). Igneous rocks from the SCB display a restricted range of EN^ and Sri isotopic values. The rocks exhibit a juvenile isotopic character indicating derivation from a depleted mantle source with little or no interaction with evolved continental material. Geochemical and isotopic data suggest incorporation of a subduction-related component that displaced isotopic values along the mantle array and led to varying degrees of LREE enrichment. Field evidence indiqates significant assimilation of crustal rocks, which consist primarily of juvenile volcanic-arc or oceanic terrane material. Smallscale temporal isotopic fluctuations are interpreted to indicate variable degrees of assimilation through time, probably associated with the rate of magma emplacement. Significant assimilation is to be expected in a large, long-lived plutonic setting, with its inherent high geothermal gradient (DePaolo et a]. 1992; Perry et al. 1993). However, isotopic and geochemical data for Jurassic-Tertiary rocks of the southern Coast Plutonic Complex do not allow us to distinguish between direct mantle derivation of magmas followed by fractionation and crustal assimilation versus wholesale melting of mafic-arc-derived lower crust as a result of magmatic underplating, as has been suggested for the Cenozoic Chilliwack batholith (Tepper et al. 1993).

Acknowledgments We acknowledge the late R.L. Armstrong, whose work and ideas inspired this project. We also thank D.K. Ghosh, J.M. Journeay, J.W.H. Monger, J.K. Mortensen, R.R. Parrish, J.K. Russell, R. Theriault, P. van der Heyden, and G.J. Woodsworth for discussions that enhanced our understanding of Coast Belt geology and Nd-Sr isotopic systematics. This work was funded by a grant from Lithoprobe and the Natural Sciences and Engineering Research Council of Canada operating grant of R.L. Armstrong.

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Appendix: Analytical techniques Rock samples of hand-specimen size or larger were crushed in jaw and disc mills and then finely ground in a motor-driven agate mortar. Sample dissolution for Sr, Sm-Nd, and Nd was done in Krogh (1973) teflon dissolution bombs, in an HF-HNO, mixture at 190-200°C for 24 h or longer, normally with aliquots of about 250 mg. The entire solution was centrifuged and the precipitate, consisting mainly of fluorides, is used for Sr, Sm-Nd, and Nd separation. Concentrated HNO, was then added to this material and dried on a hot plate to drive off remaining HF and to convert fluorides to nitrates. Since the samples for this study were processed the University of British Columbia laboratory has adopted a 5 day dissolution procedure to ensure complete digestion of refractory phases such as zircon. Several samples were reprocessed employing this procedure. An additional variation employed for these duplicates was to dry down the HF -HN03 after dissolution rather than centrifuging and discarding this liquid. The results of the latter duplicate analyses, which also employ a 5 day dissolution time, are listed in the Sm-Nd part of this appendix.

Rb - Sr Rb and Sr concentrations were determined by replicate analyses of pressed powder pellets using X-ray fluorescence. United States Geological Survey rock standards are used for calibration; mass absorption coefficients were obtained from Mo K-alpha Compton scattering measurements. Rb/Sr ratios have a precision of 2% (la) where both concentrations exceed 50 ppm. If either concentration is below 50 ppm the ratio uncertainty is based on an uncertainty in the concentration measurement of 1 ppm. Concentrations have a precision of 5 % or 1 ppm, whichever is greater.

Sr and initial rare earth element (REE) separation was done on 100-200 mesh 50-X8 Dowex (H-form) cation exchange resin columns. After Sr was stripped with 2.5 M HCl, the REE fraction was eluted with 4.0 M HCl. Sr was taken up in water and loaded on a single Ta-filament with Ta oxide as the emission base. Sr isotopic composition is measured on unspiked samples with a VacuumGenerators Isomass 54R mass spectrometer. Measured ratios have been normalized to a 86Sr/8sSrratio of 0.1194. Repeated analysis (n = 16) of National Bureau of Standards standard SrC0, (SRM 987) during the course of the study gives a mean 87Sr/86Sr ratio 110 ppm below that of the reference value of 0.71019 0.00002. Measured 87Sr/86Srratios (not adjusted) are reported in Table 2. A Rb decay constant of 1.42 x 10-I'a-' (Steiger and Jager 1977) is used for calculating initial ratios. Blanks for Rb and Sr are approximately 0.8 and 6 ng, respectively.

+

Sm - Nd Initial separation of REE was done on the same 100-200 mesh Dowex 50-X8 ion exchange resin columns used for Sr separation with 2.5 M HCl. After Sr was eluted the REE are stripped with 4 M HC1. For isotope dilution analyses a mixed '49Sm-15%d spike was added before dissolution (Andrew et al. 1991). These REEs are ready for mass spectrometry after elution through the first set of columns. The dried heavy REE fraction was loaded with water on the side filament of a double-filament bead (both filaments are Re) and the mixed REE spectrum is run on the mass spectrometer. Replicate analyses of samples from the current study, including those employing a 5 day dissolution time, demonstrate that Sm and Nd concentration reproducibility is 2 % and '47Sm/144Ndreproducibility is < 2 % . For isotope composition runs Nd is separated from Sm and other elements using columns containing 4 cm of powdered teflon, treated with Di-2-ethylhexyl orthophosphoric acid (HDEHP) and capped with 0.5 cm anion exchange resin. Nd is stripped with 0.25 M HCl, dried, and loaded with water on the Ta side filament of a doublefilament bead (the centre filament is Re). Isotope ratio measurements ('44Nd/'43Nd, corrected for l4"Sm interference via L47Sm, and '46Nd1'43Nd) are repeated alternately until a l a isotope ratio precision of 20 ppm or better is obtained, if possible. Repeated runs of the LaJolla Nd standard gives a weighted mean of (0.511847 0.00014, 20; n = 4). Measured, uncorrected '44Nd/'43Nd ratios are reported in this study. Replicate analyses of samples, including those employing a 5 day dissolution time, indicate a reproducibility of 1% or better for '43Nd/'44Nd ratios. Blanks for Sm and Nd are approximately 0.8 and 4 ng, respectively. A '47Sm decay constant of 6.54 x 10-'2a-' has been used to calculate initial Nd compositions.

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