Plutonic Evolution of the Canadian Cordillera

PETER PETÖ Geology Department, Manchester University, Ml 3 9PL, England

ABSTRACT these grade laterally into greenschist and lower facies rocks (Monger and Hutchison, 1970). The eastern belt, which is known The plutonic evolution of the Canadian Cordillera was continu- as the Cassiar-Omineca-Columbia belt (geanticline), separates a ous from Late Triassic to Miocene time, with periods of climactic continental (miogeosynclinal) sedimentary assemblage to the east plutonism during Middle Jurassic, Upper Cretaceous, and Eocene from an oceanic (eugeosynclinal) igneous and sedimentary assem- times. Although plutonism has been recurrent along extensive blage to the west. The western, predominantly plutonic belt is tracts of the Cordillera, the focus progressively migrated ocean- known as the Coast Mountains belt. In the central Yukon, the two ward from the Intermontane belt during Late Triassic time to the main plutonic belts merge and appear to have undergone consider- Insular belt during late Tertiary time. The composition of the able right-lateral displacement along two major southeasterly plutonic rocks is strongly associated with geologic age. Moore and trending fault zones. In southern British Columbia, the main others' (1963) quartz diorite line reflects predominantly plutonic belts combine into an easterly trending belt that continues granodioritic plutonism along the central Cordillera during the into the plutonic belt of the American northwest. Jurassic and Cretaceous Periods and reflects quartz dioritic plutonism along the Coast Mountains belt during early Tertiary AGE DISTRIBUTION time. Rb, Sr, Rb/Sr, K/Rb, and initial Sril7/Sr86 values of intrusions Plutonism in the Cordillera is predominantly Mesozoic and suggest origins related to different magmatic sources. Batholiths Cenozoic in age, although sporadic plutonism occurred during the along the oceanic belts have initial Sr87/Sr86 values similar to conti- late Proterozoic of southeastern British Columbia (Gabrielse and nental basalt, chondritic K/Rb and Rb/Sr values, and low Ca/Sr val- Reesor, 1964; Ryan and Blenkinsop, 1971), during the Paleozoic of ues. Intrusions along epicontinental belts have high initial Sr87/Sr86, southeastern Alaska (Lanphere and others, 1964, 1965), northern Rb/Sr, and low K/Rb values. It is suggested that intrusions along Yukon (Wanless and others, 1966), and the southeastern Rocky oceanic (eugeosynclinal) belts were generated in a marginal arc- Mountains belt (Gabrielse and Reesor, 1964; Baadsgaard and trench framework, whereas those along the epicontinental others, 1961). (miogeosynclinal) belts were generated by anatexis of older conti- Figure 2 is a histogram of K/Ar ages constructed from 394 age nental crust. Key words: geochronology, geochemistry. determinations obtained from Lowdon (1960, 1961), Lowdon and others (1963), Wanless and others (1966, 1967, 1968a, 1970), INTRODUCTION White and others (1968, 1970), Richards and White (1970), Monger and others (1972) and Dercourt (1972) presented a Nguyen and others (1968), Christopher and others (1972), Rod- plate tectonics model to explain the geologic evolution of the dick and Farrar (1972), Roddick and others (1972), Lanphere and Canadian Cordillera. Some spatial, temporal, and compositional others (1964, 1965), Loney and others (1967), and Forbes and En- considerations pertaining to the plutonic evolution of the Canadian gels (1970). Cordillera are elaborated in this paper. Plutonism in the Canadian If K/Ar dates indicate crystallization ages, then plutonism has Cordillera has been studied previously by Baadsgaard and others been continuous from Late Triassic to late Tertiary time. Based on (1961), Gabrielse and Reesor (1964), Douglas and others (1970), a compilation of 1,224 age dates, Gilluly (1973) concluded that the and Wheeler and Gabrielse (1972). duration of siliceous magmatism was continuous and episodic in The distribution of plutonic rocks in the Canadian Cordillera is the North American Cordillera during Mesozoic and Cenozoic largely confined to batholithic intrusions that occupy a considera- eras. However, Lanphere and Reed (1973), using only concordant ble area of the Cordillera (Fig. 1). Cordillerian batholiths are usu- K/Ar mineral pairs, concluded that North American plutonism was ally elongate and generally conform to the principal tectonic trends episodic but discontinuous. of the region. Monger and Hutchison (1970) have divided the The frequency distribution in Figure 2 is broadly bimodal, with Canadian Cordillera into five tectonic belts (Fig. 1). Although in- one maximum from 40 to 100 m.y. that has peaks at 45 ± 5, 75 ± trusions extensively pervaded the principal tectonic belts of the 5, and 95 ± 5 m.y. and a second maximum at 165 ± 5 m.y. The Cordillera, they are concentrated along two prominent northwest- peaks (modes) increase slightly with decreasing age. The positions trending regional metamorphic belts. The axial regions of the belts of the frequency maxima may be biased by the choice of frequency are characterized by metamorphic rocks of the amphibolite facies; intervals and by the nonrandom sampling of rocks for K/Ar age de-

Geological Society of America Bulletin, v. 85, p. 126i)-1276, 6 figs., August 1974

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Figure 1. Distribution of plutonic rocks within British Columbia, Yukon Territory, YUKON and southeastern Alaska. Intrusions represented by stippled enclosures. Areas of amphibolite and greenschist fades of regional metamorphism (after Monger and Hutchi- son, 1971) represented by horizontal ruling. Intrusions listed in Table 2 numerically designated as follows: 1. Coast Mountains batholith, 2. Guichon batholith, 3. Sirail- kameen batholith, 4. Vernon stock. 5. Nelson batholith, 6. Hellroaring Creek stock, 7. Bayonne batholith, 8. White Creek batholith, and 9. Hogem batholith. Major tectonic belts of the Canadian Cordillera shown on inset map at center right: 1. Insular belt, 2. Coast Mountains belt, 3. Intermoritane belt, 4. Cassiar-Omineca-Columbia belt, and 5. Rocky Mountains belt.

— ; ; > termination. Despite these possible biases, the frequency maxima correspond to periods of widespread plutonism along the length of the Canadian Cordillera during Middle Jurassic (165 m.y.), Upper Cretaceous (95 to 75 m.y.), and Eocene (45 m.y.) times. These periods are thought to be times of climactic plutonism. Periods of episodic plutonism in the Canadian Cordillera are also evident elsewhere in the North American Cordillera. For example, the Middle Jurassic intrusive epoch was prominent in southeastern Alaska (Reed and Lanphere, 1973), in northern California (Evern- den and Kistler, 1970), and in the American southwest (Armstrong and Suppe, 1973). The Late Cretaceous intrusive epoch appears to have occurred along the entire length of the North American Cor- dillera from Alaska and Canada to the American Cordillera (Tilling and others, 1968; McDowell and Kulp, 1969; Evernden and Kis- tler, 1970; and Armstrong and Suppe, 1973). However, the Eocene intrusive epoch was largely confined to the Canadian Cordillera, although Eocene intrusions are evident in Idaho and in the Basin and Range region. Gilluly (1973) concluded that plutonism need not be associated with orogeny. However, in the Canadian Cordillera, the frequency distribution of K/Ar ages of plutonic rocks is broadly correlative with the Nassian (mid-Jurassic), Columbian (Upper Cretaceous), and Laramide (early Tertiary) orogenies (Douglas and others, 1970).

PLUTONIC EVOLUTION The spatial distribution of the K/Ar age data indicates that plutonism recurred along extensive tracts of the Cordillera over long periods of time. This is evident from the intrusive history of areas, such as the central Coast Mountains (Hutchison, 1970), southeastern British Columbia (Gabrielse and Reesor, 1964), the Cassiar Mountains (Christopher and others, 1972), and the regions near Hope (Richards and White, 1970) and Hedley (Roddick and others, 1972). Aside from the persistence of plutonism in local areas, a regional trend indicates that the focus of plutonism migrated progressively westward from the interior of British Colum- bia during the Late Triassii: to the Insular belt during the late Ter-

ao 100 120 KILOMETERS K/Ar AGES (M.Y.) 0 100 200 300 Figure 2. Frequency histogram of K/Ar ages from plutonic intrusions of Canadian 100 0 100 Cordillera. MILES

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Tertiary ceous Jurassic dioritic in composition, whereas those rocks in the eastern Cordil- Cre,â 180 Tria«ic 70 lera are largely granodioritic and quartz monzonitic. Moore and -20 COASTA . BELTS others (1963) outlined a sharp line between the two compositional provinces, which they termed the "quartz diorite boundary line."

-10 Moore (1959) believes that this line represents a fundamental compositional difference in the crustal source from which the intrusive r hj rocks were generated. The location of the "quartz diorite line" along the Coast Mountains belt is problematic due to abundant in- INTERMONTANE BÍ:LT trusions of acidic rock scattered in a region consisting predomi- -10 nantly of quartz diorite and granodiorite (Hutchison, 1970). Table 2 gives the distribution of rock types in the plutonic belts of the Canadian Cordillera. Quartz diorite is most abundant in the coastal belts, whereas quartz monzonite and granite predominate CASSI AR—OMIN EC A—COI.UMB IA in the Cassiar-Omineca-Columbia belt. Granodiorite occurs with -20 BE LT equal abundance in each belt. Tables 1 and 2, therefore, indicate that Moore's "quartz diorite line" does not solely delimit a spatial -10 Hl H separation, but also a temporal separation, in the distribution of quartz diorite in the Canadian Cordillera.

AGE (M Y) TABLE 2. DISTRIBUTION OF INTRUSIVE ROCK TYPES IN THE CORDILLERA Figure 3. Age distribution of plutonic rocks in ti.-ctonic belts of Canadian Cordillera. Others Granite Quartz Grano- Quartz Diorite Sample monzonite diorite diorite total*

tiary. The plutonic evolution of the Canadian Cordillera is illus- Coastal belts 6 8 25 55 55 11 160 Intermontane 8 5 21 18 18 2 72 trated in Figure 3. The general east-to-west intrusive trend, how- belt ever, shows a local reversal in the Coast Mountain belt (Hutchison, Cassiar-Omineca- 15 25 51 49 5 15 160 Columbia belt 1970), which shows an intrusive sequence similar to that of the Coastal belt of northern Chile (Farrar and others, 1970). Rock totals 29 38 97 122 78 28 392*

Sporadic plutonism occurred along the Intermontane belt during * Abundance of each rock type compiled from age data used in Table 1, including Late Triassic time and extended into the Cassiar-Omineca- intrusive rocks older than 190 m.y. Columbia belt during the Jurassic period. Middle Jurassic plutonism also occurred along the Insular belt; this may not have been contiguous with the Cordillera during Triassic time. Berry and RB, SR, RB/SR, K/RB, AND 87 86 others (1971) suggested that Vancouver Island was at a SR /SR DISTRIBUTIONS paleolatitude much farther south than at present. There appears to Petrochemical data of some Cordilleran intrusions are listed in be a peculiar plutonic hiatus'along regions of the Insular belt be- Table 3; the locations of intrusions are shown in Figure 1. The data tween the Jurassic period and Eocene epoch. Intermontane suggest that intrusions that have relatively low Rb, Rb/Sr, Sr87/Sr86, plutonism spread westward into the Coast Mountains belt during and high K/Rb and Sr are along the oceanic (eugeosynclinal) belts the Early Cretaceous, and plutonism extended into the Insular belt of the western Cordillera, whereas the intrusions with higher Rb, at the beginning of the early Tertiary. During the late Tertiary, Rb/Sr, Sr87/Sr8S, and low K/Rb and Sr values are nearer the conti- plutonism became confined to the Insular and Coast Mountains nental (miogeosynclinal) wedge of the eastern Cordillera. The pro- belts and to portions of southern British Columbia. nounced increase in the amount of radiogenic Sr from the Coast Mountains batholith to the Hellroaring Creek stock is concomitant COMPOSITIONAL TRENDS with a decrease in K/Rb values. Table 3 also indicates that the Cordilleran batholiths consist largely of intermediate plutonic Coast Mountains, Similkameen, White Creek, Hogem, and rocks. Granodiorite is most abundant, with more acidic and basic Guichon batholiths have rather high Sr contents as compared to Sr rocks progressively less abundant (Table 1). Table 1 gives the contents in other igneous rock types (Fig. 4). Sr enrichment is par- number of samples which represent the common plutonic rock ticularly conspicuous in relation to Ca. A compilation of unpub- types grouped in intervals of 20 m.y. A contingency test on the data lished Ca/Sr data from the Similkameen, Hogem, and Guichon listed in Table 1 indicates that rock composition is associated with batholiths give average values of 60, 35, and 50, respectively. These geologic age at the 99.5 percent confidence level. Quartz diorite is values are much lower than the average Ca/Sr values (154 for predominant in Tertiary rocks, whereas granodiorite and quartz basalts and 166 for argillites) reported by Turekian and Kulp monzonite are predominant in Cretaceous rocks. (1956). The K/Rb values of intrusions in the western Cordillera are Moore (1959) and Moore and others (1963) suggested that similar to those of ordinary chondrites (350 ± 50) and inter- plutonic rocks in the western Cordillera are predominantly quartz mediate to average K/Rb values of oceanic and continental crust. Rb and Sr values in the batholithic rocks are similar to those of alkali basalts and andesites and are much higher than those of tholeii- TABLE 1. CONTINGENCY TABLE OF ROCK TYPE VERSUS INTRUSIVE AGE tic basalts (Fig. 4). The Rb and Sr abundance and the Rb/Sr ratio of Rock sample K/Ar age interval (m.y. > Rock an average Cordilleran granodiorite are intermediate between dif- frequency 10 30 50 70 90 110 130 150 170 190 total ferentiated crustal and upper mantle derivatives, and they are simi- Quartz diorite 7 21 9 11 5 9 8 11 5 86 lar to those of composite crust (Fig. 4). Granodiorite 6 10 15 21 2) 9 12 13 6 116 Quartz monzonite 8 9 16 16 17 6 6 13 2 93 Granite 3 9 8 4 4 1 3 3 4 39 Others* 2 8 0 0 2 5 0 6 3 26 DISCUSSION Age totals 26 57 48 52 E2 30 29 46 20 360 An adequate hypothesis that will account for the phenomenon of * Other rock types, in order of relative abundance, include diorite, gabbro, Cordilleran plutonism must explain the generation of large quan- syenite, monzonite, and monzodiorite. tities of compositionally diverse magmas over vast areas of the

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TABLE 3. Rb, Sr. Rb/Sr, K/Rb, and Sr"/Sr" ABUNDANCES OF SOME CORDILLERAS INTRUSIONS

Intrusion Rb Sr Rb/Sr K/Rb Sr87/Sr86 Age References (ppm) (ppm) (m.y. )

Coast Mountains 33 725 0.046 373 0.7038* 140-84 Culbert, 19!'2 batholith (150-4) (795-25) (0.60-0.11) (225-628) (0.7031-0.7050) 79-64 Hutchison, '970 50-40 Similkameen 95 390 0.151 250 0.7060* 183, Petö, 1973 batholith (152-52) (639-147) (1.01-0.081) (309-172) (0.7029-0.7091) 160-140 Fairbairn ard others, 1964 Roddick and others, 1972 White Creek 265 804 0.412 0.7250+ 126,111 Wanless and others, 1968b batholith (357-196) (1118-435) (1.655-0.108) (0.7076-0.7397) Hellroaring Creek 446 23 59.3 121 0.81 ± 0.06+ 1,260 ± 50 Ryan and ßlerikinsop, 1971 stock (1,058-70) (47-9) (170-2) (270-63) (9.919-0.8366) Hogem 80 730 0.100 430 170 Meade, 1972 batholith (118-55) (1,520-468) (0.125-0.041) (502-322) White and others, 1968 Nelson 0.175 0.7069* 171-49 Fairbairn and others, 1964 batholith (0.483-0.056) (0.7057-0.7085) Gabrlelse ant' Reesor, 1964 Bayonne 0.246 0.7081* 118, 110 dc. batholith (0.357-0.115) (0.7072-0.7090) Vernon 1.42 0.7064+ 55 do. batholith (2.84-0.108) Guichon 620 0.7037+ 200 ± 5 White and oth?rs, 1967 batholith Chrismas and others, 1969 Brabec, 1971

Note: Values in parentheses indicate observed range. * Mean initial Sre7/SrBi calculated from present Sr°7/Sr06 and apparent K/Ar age. t Initial Srf7/Sre6 obtained from I'.b/Sr isochron.

Cordillera throughout long periods of time. With the advent of the high Sr abundances; this suggests derivation from either the vol- theory of plate tectonics, Cordilleran plutonism can be considered canic archipelago or the continental upper mantle. Intrusions, such to be the result of geodynamic processes associated with plate con- as the Coast Mountains and Guichon batholiths, have initial sumption along convergent plate boundaries. The efficacy of gen- S.r87/Sr86, Rb/Sr, and Rb values similar to oceanic basalts; this sug- eration of calc-alkaline magma along active zones of subduction gests derivation from subducted oceanic crust or upper mantle. was recognized by Green and Ringwood (1968) and subsequently Faure and Powell (1972) report that Cordilleran batholiths have applied to the tectonic evolution of the North American Cordillera an average initial Sr87/Srw value of 0.707 ± 0.001, which is by Hamilton (1969). Monger and others (1972) applied plate tec- significantly higher than that of modern ocean floor basalts (0.7037 tonics concepts to the geologic evolution of the Canadian Cordil- ± 0.001) and considerably lower than that of cratonic crust lera. According to their model, sea-floor spreading and subduction (0.719). The similarity of initial Sr87/Sr86 values of batholiths to of Pacific oceanic crust along the Cordillera from Upper Triassic to those of continental basalts, reported by Leeman and Manton late Tertiary time resulted in the linkage of allochthonous crustal (1971), suggests derivation from a common isotopic regime. The segments of diverse provenance to the leading edge of the Canadian slightly higher Sr87/Sr86 value of batholiths generally is attributed to craton. The phenomenon of Cordilleran plutonism is attributed by contamination by more radiogenic crustal material. However, an Dickinson (1970) to magmatisrn associated with the consumption alternative hypothesis involves (1) fractionation of subcontinental of oceanic crust along a subduction zone within an arc-trench mantle to produce an upper zone enriched in Rb87 relative to Sr86 framework. Assuming that Mesozoic and Cenozoic plutonism of and a residual lower zone depleted in Rb87 relative to Sr86, and (2) the Canadian Cordillera occurred in an arc-trench framework, one generation of batholithic magma from the Rb-enriched upper zone may postulate that (1) plate consumption continued along vast and generation of continental basaltic magma from the Rb- stretches of the Cordillera for at least 200 m.y.; (2) plate margins depleted lower zone. If Rb enrichment occurred with sufficient time have migrated progressively oceanward with the addition of newly to generate a significant amount of radiogenic Sr before generation formed crust to the leading edge of the Canadian craton; (3) the of the batholithic and continental basaltic magma, the batholiths intensity of plutonism is related to the rate of plate convergence wojld have a higher Sr87/Sr86 value than the continental basalts of and plate consumption; and (4) the composition of plutonic mag- the same age (Fig. 6). matisrn has varied with the age and position of convergent plate Figure 6 schematically illustrates the evolution of S::87/Sr86 in the junctures. upper mantle throughout geologic time. Mesozoic ba tholiths, de- Tectonic and compositional considerations, based on the data in rived from basic source rocks in the uppermost mantle, would have Figure 1 and Table 3, indicate that Cordilleran intrusions have di- higher SrS7/Sr86 values than basalts derived from oceanic mantle or verse origins related to magmatic provenance. In terms of a continental lower mantle depleted in Rb87 prior to Mesozoic time. hypothetical arc-trench subduction model across the Canadian Peterman and others (1967! suggested that anatexis of Mesozoic Cordillera, several plausible magmatic sources are illustrated in and Cenozoic graywackes could produce the Cordilleran Figure 5. These sources are (1) subducted oceanic crust, (2) vol- batholithic magmas with appropriate Sr87/Sr86 values. However, the canic rocks in island archipelagos, (3) miogeosynclinal metamor- Rb and Sr abundances they reported appear somewhat higher than phic rocks along the edge of the Precambrian craton, (4) subconti- those of eugeosynclinal volcanic rocks; this suggests that plutonic nental upper mantle, and (5) cratonic basement gneiss and granu- detritus may comprise a large clastic portion of Cordilleran lite. For example, the rather high Sr87/Sr86, Rb, Rb/Sr, and low K/Rb graywackes. Also, it is doubtful whether sufficient quantities of and Sr values of the Hellroaring Creek stock and the younger intru- graywacke were available in the volcanic pile to generate the sions of the White Creek batholith suggest derivation by partial fu- enormous quantity of plutonic rocks in the Cordillera. Further- sion of micaceous Precambrian metasediments or basement gneiss. more, anatexis of a micaceous graywacke probably would produce Intrusions into Upper Triassic volcanic rocks of the Intermontane a granitic melt (Winkler, 1967), having low Sr and K/Rb values and belt have initial Sr87/Sr86 ratios similar to those of continental high K, Rb, and Rb/Sr values, which would be inappropriate to the basalts, chondritic K/Rb and Rb/Sr values, and intermediate Rb and composition of most eugeosynclinal Cordilleran intrusions. Kilinc

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1000 dilleran intrusions. Also, composite intrusions may result from multiple melting episodes involving different degrees of partial fusion (Presnall and Bateman, 1973). Granites, for example, may be derived by very small amounts of fractional fusion, whereas dio- rites may be derived by more extensive fusion of basic source rocks. Rb, Sr, Rb/Sr, K/Rb, and Sr87/Sr86 values of eugeosynclinal batholithic intrusions preclude derivation from upper crustal, metamorphic, and sedimentary differentiates. Direct derivation by partial fusion of mantle peridotite is unlikely, as only infinitesimal amounts of compositionally appropriate melt could be generated from peridotite, as enrichment factors that approach two orders of magnitude are required. Similarly, partial fusion of tholeiitic ocean-floor basalt, subducted beneath continental margins, appears implausible insofar as the low Rb, Sr, Rb/Sr, and high K/Rb values in these rocks imply that very small degrees of fusion are required to produce appropriate melts. The enormous amounts of eugeosynclinal volcanic rock that erupted along the continental margin, however, may provide an adequate magmatic source. Rb, Sr, Rb/Sr, and K/Rb values of andesites and alkali basalts suggest that fusion of up to 50 percent of these rocks could produce melts with an appropriate petrochemistry. It is suggested that eugeosynclinal batholithic magmas could be generated from these rocks at relatively shallow depths along continental margins in an orogenic thermal regime.

CONCLUSIONS The plutonic evolution of the Canadian Cordillera from the Upper Triassic to late Tertiary time may be as follows. Epiconti-

Sr(ppm) 1000

Figure 4. Average Rb and Sr abundances. Rock types abbreviated: Gr = granite, Gd = granodiorite, Pr = peridotite, Dn = dunite, Ry = rhyolite, Ad = andesite, AB = alkali basalt, OT = oceanic tholeiite, OC = ordinary chondrite, pC = Precambrian basement, Sh = shale and Gw = graywacke. Abundance estimates obtained from Faure and Powell (1972), Armstrong (1968), Burv.ash and others (1973), Wedepohl (1968).

(1972) showed that partial fusion of graywacke in a saline solution produced a sodic melt similar in composition to trondhjemite. The chemical composition of Cordilleran intrusions need not en- tirely be determined by the composition, mineralogy, and degree of partial fusion of the source rock but also may be modified during ascent and crystallization of the magma. Distribution coefficients, reported by Philpotts and Schnetzler (1970), indicate that fractionation of plagioclase would tend to increase the Rb, K, Rb/Sr, and Ca/Sr and would therefore lower the Sr and K/Rb values of derivative liquids. Amphibole fractionation would tend to increase Rb, Sr, and Rb/Sr values and lower K, K/Rb, and Ca/Sr values of residual liquids. Therefore, crystal fractionation may be an efficient mechanism for producing the diversity of intrusive phases in Cor-

0.699

TIME(X 10 yr.)

Figure 6. Schematic SrH7/SrH,i evolution diagram. Solid lines represent inferred Sr87/Srse growth curves of continental upper mantle (C.U.M.), oceanic mantle (O.M.), and Archean crust with geologic time. Dashed lines indicate mean linear growth rates Figure 5. Hypothetical section across Canadisji Cordillera in terms of arc-trench of Srs7/SrtM in continental basalts or Cordilleran batholiths (C.B.) and oceanic tholeiite subduction model. Plausible magmatic sources for Cordilleran intrusions include: 1. (O. T.). Assumed that mantle began to differentiate Rb87 about 2.5 B.Y. ago. Cordille- subducted oceanic crust, 2. volcanic rocks benea:h continental island arcs, 3. upper ran batholiths thought to have been derived by partial fusion of basic rocks derived mantle, 4. epicontinental metasedimentary rocks, and 5. cratonic basement gneiss. from continental upper mantle prior to Mesozoic.

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nental island-arc volcanism along the Intermontane belt during Burwash, R. A., Krupicka, J., and Culbert, R. R., 1973, Cratonic reactiva- Upper Triassic time probably indicates-the beginning of active sub- tion in the Precambrian basement of western Canada. III. Crustal duction of Permian-Triassic oceanic crust toward the margin of the evolution: Canadian [our. Earth Sci., v. 10, p. 283-291. Chrismas, L., Baadsgaard, H., Folinsbee, R. E., Fritz, P., Krouse, H. R., and Canadian craton. The initiation of plate convergence along the Sasaki, A., 1969, Rb/5r, S and isotopic analyses indicating source and margin of the American plate may be caused by incipient rifting date of contact meta: omatic copper deposits, Craigmont, British Co- along the Pacific Ocean ar.d (or) westward drift of the American lumbia, Canada: Economic Geology, v. 64, p. 4'79-488. plate. The focus of Upper Triassic volcanism in relation to the Christopher, P. A., White, W. H„ and Harakal, J. E., 1972, Age of molyb- highly deformed metasedimentary pile of the Cassiar-Omineca- denum and tungsten mineralisation in northern British Columbia: Columbia belt suggests thai: Upper Triassic subduction was largely Canadian Jour. Earth Sci., v. 9, p. 1727-1734. determined by the leading edge of the Canadian craton. Triassic in- Culbert, R., 1972, Abnormalities in the distribution of K, Rb, and Sr in the trusions were emplaced along volcanic archipelagos. Widespread Coast Mountains bi.tholith, British Columbia: Geochim. et Cos- siliceous plutonism extended into the Cassiar-Omineca-Columbia mochim. Acta, v. 36, p. 1081-1100. belt, where it reached a climax during Middle Jurassic time. Burial Dercourt, J., 1972, The Canadian Cordillera, the Hellenides, and the sea-floor spreading theory: Canadian Jour. Earth. Sci., v. 9, p. metamorphism of thick Precambrian and Lower Cambrian shelf 709-743. deposits coincided with extensive magmatism and a resultant in- Dickinson, W. R., 1970, Relation of andesites, granites, and derivative crease in the regional geothermal gradient. However, active plate sandstones to arc-trench tectonics: Rev. Geophys. Space Phys., v. 8, p. convergence migrated oceanward by the end of Jurassic time, 813-860. perhaps in response to the addition of substantial amounts of igne- Douglas, R.J.W., Gabrielse, H., Wheeler, J. O., Stott, D. F., and Belyea, H. ous and metamorphic crust to the cratonic margin. Plutonism ex- R., 1970, Geology of western Canada, in Geology and economic min- tended into the Coast Mountains belt at the beginning of Lower erals of Canada: Canada Geol. Survey Econ. Geology Rept. 1, p. Cretaceous time, while the Cassiar-Omineca-Columbia belt was 366-488. undergoing isostatic uplift and denudation in response to the cessa- Evemden, J. F., and Kistle::, R. W., 1970, Chronology of emplacement of tion of crustal subduction in that region. Active plate consumption Mesozoic batholithic complexes in California and western Nevada: U.S. Geol. Survey Prof. Paper 623, 42 p. and plutonism reached a climax along the central Cordillera by Fairbairn, H., Hurley, P., and Pinson, W., 1964, Initial Sr87/Sr86 and possi- Late Cretaceous time (Larsen and Pitman, 1972). However, at the ble sources of granitic rocks in southern British Columbia: Jour. beginning of the Tertiary, active subduction of Pacific oceanic crust Geophys. Research, v. 69, p. 4889-4893. had migrated westward, perhaps in response to the arrival of Farrar, E., Clark, A. H., Haynes, S. J., Quirt, G. S., Connand, H., and Zen- allochthonous crustal fragments, which were rafted against the tilli, M., 1970, K-Ar evidence for the post-Paleozoic migration of Cordillera from the eastward-moving Pacific oceanic plate along granitic intrusion foci in the Andes of northern Chile: Earth and the Insular belt (Berry and others, 1971; Dercourt, 1972; Monger Planetary Sci. Letters, v. 10, p. 60-66. and others, 1972). Widespread quartz dioritic plutonism during Faure, G., and Powell, J. L., 1972, Strontium isotope geology: New York, Eocene- time occurred along the Insular belt and was associated Springer-Verlag, 188 p. 40 40 with extensive continental basaltic volcanism along the Intermon- Forbes, R., and Engels, J., .970, K /Ar age relations of the Coast Range batholith and related rocks of the Juneau ice field, Alaska: Geol. Soc. tane belt, regional metamorphism along the Omineca and Colum- America Bull., v. 81, p. 579-584. bia belts, and intense overthiusting of miogeosynclinal sedimentary Gabrielse, H., and Reesor, J., 1964, Geochronology of plutonic rocks in rocks along the Rocky Mountains belt. These events coincided with two areas of the Canadian Cordillera: Royal Soc. Canada Spec. Pub., a period of rapid sea-floor spreading and consumption of Pacific v. 8, p. 96-138. oceanic crust (Larsen and Pitman, 1972). At the beginning of late Gilluly, J., 1973, Steady plate motion and episodic orogeny and mag- Tertiary time, plutonism was confined to a narrow zone along the matism: Geol. Soc. America Bull., v. 84, p. 499-514. Insular and Coast Mountains belts and to southern British Colum- Green, T. H., and Ringwood, A. E., 1968, Genesis of the calc-alkaline igne- bia. The cessation of active subduction of oceanic crust resulted in ous rock suite: Contr. Mineralogy and Petrology, v. 18, p. 105-162. gradual regional uplift and denudation of deeply buried crust along Hamilton, W., 1969, Mesozoic California and the underflow of Pacific the Coast Mountains belt. Miocene plutonism was generally mantle: Geol. Soc. America Bull., v. 80, p. 2409-2*30. confined to the Insular belt, suggesting progressive cessation and Hutchison, W. W., 1970, Metamorphic framework and plutonic styles in the Prince Rupert region of the central Coast Mountains, British Co- oceanward retreat of active subduction from the Eocene to Pliocene lumbia: Canadian Jour. Earth Sci., v. 7, no. 2, p. 376^105. periods. Kilinc, I. A., 1972, Experimental study of partial melting of crustal rocks and formation of migitiatites, in Petrology-Petrologie, Sect. 2: Inter- nat. Geol. Cong., Proc.. no. 24, p. 109-113. ACKNOWLEDGMENTS Lanphere, M. A., and Reed, B. L., 1973, Timing of Mesozoic and Cenozoic The manuscript was improved by suggestions from John Payne plutonic events in circi.m-Pacific North America: Geol. Soc. America and J. G. Moore. Bull., v. 84, p. 3773-3782. Lanphere, M. A., MacKevert, E. M., and Stern, T. W., 1964, Potassium- argon and lead-alpha ages of plutonic rocks, Bokan Mountain area, REFERENCES CITED Alaska: Science, v. 145, p. 705-707. Armstrong, R. L., 1968, A model for the evolution of strontium and lead Lanphere, M. A., Loney, R. A., and Brew, D. A., 1965, Potassium-argon isotopes in a dynamic earth: Rev. Geophysics, v. 6, p. 175-199. ages of some plutonic rocks, Tenakee area, Chickhagof Island, south- Armstrong, R. L., and Suppe, J., 1973, Potassium-argon geochronometry eastern Alaska: U.S. Geal. Surv. Prof. Paper 525B, p. B108-B111. of Mesozoic igneous rocks in Nevada, Utah, and southern California: Larsen, L. L., and Pitman, W. C., 1972, World-w.de correlation of Geol. Soc. America Bull., v. 84, p. 1375-1392. Mesozoic magnetic anomalies and its implications: Geol. Soc. Baadsgaard, H., Folinsbee, R. F.., and Lipson, J., 1961, Potassium-argon America Bull., v. 83, p. 3645-3662. dates of biotite from cordiLeran granites: Geol. Soc. America Bull., v. Leeman, W. R., and Manton, W. I., 1971, Strontium isotopic composition 72, p. 689-701. of basaltic lavas from i:he Snake River Plain, southern Idaho: Earth Berry, M. J., Jacoby, W. R., Niblett, E. R., and Stacey, R. A., 1971, A re- and Planetary Sci. Letters, v. 11, p. 420-434. view of geophysical studies in the Canadian Cordillera: Canadian Loney, R. A., Brew, D. A., and Lanphere, M. A., 1967, I'ost-Paleozoic ages Jour. Earth Sci., v. 8, no. 7, p. 788-801. and their relevance to fault movements, northern southeastern Alaska: Brabec, D., 1971, A geochernic.il study of the Guichon Creek batholith, Geol. Soc. America Bull , v. 78, p. 511-526. British Columbia [Ph.D. thesis]: Univ. British Columbia, 158 p. Lowdon, J. A., 1960, Isotopic ages, Rept. 1 of Age determinations by the

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Geological Survey of Canada: Geol. Survey Canada Paper 60-17, Roddick, J. C., Farrar, E., and Procyshyn, E. L., 1972, Potassium-argon 51 p. areas of igneous rocks from the area near Hedley, southern British 1961, Age determinations by the Geological Survey of Canada: Geol. Columbia: Canadian Jour. Earth Sci., v. 9, p. 1632-1639. Survey Canada Paper 61-17,127 p. Ryan, B., and Blenkinsop, J., 1971, Geology and geochronology of the Lowdon, J. A., Stockwell, C. H., Tipper, H. W., and Wanless, R. K., 1963, Hellroaring Creek stock, British Columbia: Canadian Jour. Earth Sci., Age determinations and geological studies: Geol. Survey Canada v. 8, p. 85-95. Paper 62-17,140 p. Tilling, R. I., Klepper, M. R., and Obradovich, J. D., 1968, K-Ar ages and McDowell, F. W., and Kulp, J. L., 1969, Potassium-argon dating of the time span of emplacement of the Boulder batholith, Montana: Am. Idaho batholith: Geol. Soc. America Bull., v. 80, p. 2379-2382. Jour. Sci., v. 266, p. 671-689. Meade, H., 1972, A study of the differentiation trends of the Hogem Turekian, K. K., and Kulp, J. L., 1956, The geochemistry of strontium: batholith, northern British Columbia [B.Sc. thesis]: Univ. British Co- Geochim. et Cosmochim. Acta, v. 10, p. 245-296. lumbia, 52 p. Wanless, R. K., Stevens, R. D., Lachance, G. R., and Rimsaite, J.Y.H., Monger, J. W., and Hutchison, W. W., 1971, Metamorphic map of the 1966, Age determinations and geological studies: Geol. Survey Canadian Cordillera: Geol. Survey Canaca Paper 70-33, 61 p. Canada Paper 65-17, 126 p. Monger, J. W., Souther, J. G., and Gabrielse, H., 1972, Evolution of the Wanless, R. K., Stevens, R. D., Lachance, G. R., and Edmonds, C. M., Canadian Cordillera: A plate tectonic model: Am. Jour. Sci., v. 272, p. 1967, Age determinations and geological studies: Geol. Survey 577-602. Canada Paper 66-17, 120 p. Moore, J. G., 1959, The quartz diorite boundary line in the western United 1968a, Age determinations and geological studies: Geol. Survey States: Jour. Geology, v. 67, p. 198-210. Canada Paper 67-2, pt. A, 141 p. Moore, J. G., Grantz, A., and Blake, M. C., Jr., 1963, The quartz diorite Wanless, R. K., Loveridge, W., and Mursky, G., 1968b, A geochronological line in northwestern North America: U.S. Geol. Survey Prof. Paper study of the White Creek batholith, southeastern British Columbia: 450-E, p. E89-E93. Canadian Jour. Earth Sci., v. 5, p. 375-386. Nguyen, K. Sinclair, A., and Libby, W., 1968, Age of the northern part of Wanless, R. K., Stevens, R. D., Lachance, G. R., and Delabio, R. N., 1970, the Nelson batholith: Canadian Jour. Earth Sci., v. 5, p. 955-957. Age determinations and geological studies: Geol. Survey Canada Peterman, Z., Hedge, C., Coleman, R. G., and Snavely, P. D., Jr., 1967, Paper 71-2, 96 p. 87Sr/86Sr ratios in some eugeosynclinal sedimentary rocks and their Wedepohl, K. H., 1968, Chemical fractionation in the sedimentary envi- bearing on the origin of granitic magma in orogenic belts: Earth and ronment, in Ahrens, L. H., ed., Origin and distribution of the ele- Planetary Sci. Letters, v. 2, p. 433-439. ments: Oxford, Pergamon Press, p. 999-1016. Peto, P., 1973, Petrochemical study of the Similkameen batholith, British Wheeler, J., and Gabrielse, H., 1972, Variations in tectonic styles: Geol. Columbia: Geol. Soc. America Bull. v. 84. p. 3977-3984. Assoc. Canada Spec. Paper No. 11, p. 1-81. Philpotts, J., and Schnetzler, C., 1970, Phenocryst-matrix partition White, W. H., Erickson, G. P., Northcote, K. E., Dirom, G. E., and coefficients for K, Rb, Sr, and Ba with applications to anorthosite and Harakal, J. E., 1967, Isotopic dating of the Guichon batholith, British basalt genesis: Geochim. et Cosmochim. Acta, v. 34, p. 307-322. Columbia: Canadian Jour. Earth Sci., v. 4, p. 677-690. Presnall, D. C., and Bateman, P. C., 1973, Fusion relations in the sys- White, W. H., Harakal, J. E., and Carter, N. C., 1968, Potassium-argon

tem NaAlSi308-CaAl2Si208-KAlSi308-Si02-H20 and the genera- ages of some ore deposits in British Columbia: Canadian Inst. Mining tion of granitic magmas in the Sierra Nevada batholith: Geol. Soc. and Metallurgy Bull., v. 61, p. 1326-1334. America Bull., v. 84, p. 3181-3202. White, W. H., Sinclair, A., Harakal, J., and Dawson, K., 1970, Potassium- Reed, B. L., and Lanphere, M. A., 1973, Alaska-Aleutian range batholith: argon ages of Topley intrusions near Endako, British Columbia: Geochronology, chemistry and relation to circum-Pacific plutonism: Canadian Jour. Earth Sci., v. 7, p. 1172—1178. Geol. Soc. America Bull., v. 84, p. 2583-2610. Winkler, H.G.F., 1967, Petrogenesis of metamorphic rocks: New York, Richards, T., and White, W., 1970, K-Ar ages of plutonic rocks between Springer-Verlag, 237 p. Hope, British Columbia and the 49th parallel: Canadian Jour. Earth Sci., v. 7, p. 1203-1207. Roddick, J. C., and Farrar, E., 1972, Potassium-argon ages of the Eagle granodiorite, southern British Columbia: Canadian Jour. Earth Sci., v. MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 4, 1973 9, p. 596-599. REVISED MANUSCRIPT RECEIVED FEBRUARY 28, 1974

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