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Plutonic Evolution of the Canadian Cordillera

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Plutonic Evolution of the Canadian Cordillera 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 1269 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/8/1269/3418478/i0016-7606-85-8-1269.pdf by guest on 25 September 2021 1270 P. PETO Figure 1. Distribution of plutonic rocks within British Columbia, Yukon Territory, YUKON and southeastern Alaska. Intrusions represented by stippled enclosures. Areas of am- phibolite 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, al- though 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 mi- grated 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/8/1269/3418478/i0016-7606-85-8-1269.pdf by guest on 25 September 2021 PLUTONIC EVOLUTION OF THE CANADIAN CORDILLERA 1271 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 com- positional 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
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