IGNEOUS AND TECTONIC PROVINCES OF WESTERN CANADA 



585 



The Coast batholith is the largest of the Mesozoic intrusions. It forms the 



core of the Coast Range and extends northwesterly about 1,100 miles from 



I the northern part of the State of Washington to Yukon. Its width averages 



I more than 50 miles and locally exceeds 125 miles. Flanking it for many miles 



on either side are smaller, related intrusive masses that, with the rocks of 



the main batholith, comprise what is commonly known as the Coast intrusions. 



1 In southern British Columbia the batholith curves towards the east and is 



I linked with the presumably related Nelson batholith of Kootenay district by 



other intervening intrusions. The Coast intrusions range in composition from 



: granite to gabbro, but are mainly of granodiorite and quartz diorite. The 



I batholith is a composite of an unknown number of phases that were emplaced 



. as successive irruptions over a long period of time, and, presumably, the 



numerous satellitic bodies are likewise of more than one age. The younger 



I phases commonly show sharp intrusive contacts against older phases, and in 



many localities that batholithic rocks cut Lower Cretaceous sediments that 



contain pebbles of earlier batholithic rocks. It has been suggested that in 



northern British Columbia the more acid phases are most common towards 



the interior of the batholith. In the southern part of the province, however, 



the eastern intrusions, such as the Nelson batholith, are more acid and contain 



a greater proportion of granite than those nearer the coast. 



A potassium-argon age determination of the Coast Range batholith 

 near Vancouver is reported by Folinsbee et al. ( 1957 ) as 105 m.y. This 

 i is about Mid-Cretaceous on the Holmes scale. 



The eastern belt of batholiths starts southwest of the south end of the 

 Finlay River volcanic field at about Fort Frazer and Ruins Lake ( see 

 Geologic Map of Canada, 1947 ) and extends northward to the Yukon 

 and Alaska. In the Yukon the batholiths are so numerous that no marked 

 division can be noted between those of the Coast Range belt and those 

 of the eastern belt. Also at the south end of the eastern belt in the Burns 

 1 Lake region numerous small plutons bridge the Coast Range batholith 

 i to the eastern batholiths. Retween the south end and the northern cluster 

 is the Cassiar-Omineca batholith, which is over 500 miles long but rather 

 narrow. It is only partially explored and may not yet be completely 

 unroofed by erosion. 



These rocks commonly grade into one another and are not known to 

 represent more than one continuous period of intrusion. Near Takala Lake 

 the batholithic rocks cut Jurassic strata of early Upper Jurassic age. They also 

 appear to have been the source of pebbles found in the early Upper Cretaceous 



conglomerate. Thus, so far as known, the main Cassiar-Omineca batholith 



is of Upper Jurassic or Lower Cretaceous age. 



Intrusive, stock-like, tabular, and irregular bodies of serpentinized dunite, 

 peridotite, pyroxenite, and gabbro are found in southern Yukon, in Deasc Lake 

 and Takla areas of northern and central British Columbia, and in Bridge River. 

 Hope, Princeton, and other areas of the southwestern part of the province. The 

 largest are more than 100 square miles in area, but most of them are much 

 smaller. They are commonly considered to be early phases of the Mesozoic 

 batholithic intrusions and to be of Jurassic age (Lord et al. y 1947 . 



A number of batholiths in the Selwyn-Mackenzie mountains salient 

 seem to lie east of the main eastern belt, and might be thought of as 

 Laramide. Yet a potassium-argon age determination by Folinsbee et al. 

 (1957) on the Itsi batholith (Fig. 37.1) is 102 m.y. or Mid-Cretaeeous. 

 This is about the same age as the Coast Range batholith at Vancouver. 

 It seems necessary to conclude that all the scattered batholiths in this 

 salient are of the same age until further determinations are made. 



In southeastern British Columbia the Bayonne batholith immediately 

 east of Kootenay Lake has a potassium-argon age of 82 m.y. and is, there- 

 fore, about the same as the Boulder batholith of Montana to the south- 

 east which is 87 m.y. old ( Knopf, 1957 ) . Both would be Late Cretaceous 

 according to these dates and referable to the early Laramide phase of 

 orogeny. This batholith and the one just north of it, are therefore placed 

 in the Laramide belt of orogeny (see map, Fig. 37.1) and are considered 

 late satellites of the Nelson batholith and the Idaho batholith. 



POST-BATHOLITHIC VOLCANISM 



In the Canadian Cordillera crustal movements occurred during the 

 Tertiary from place to place along with considerable volcanism. The 

 Tertiary disturbances, from what is known, consisted of faulting, tilting, 

 and open folding. From Paleocene through Oligocene time sedimentary 

 and volcanic rocks accumulated in numerous small basins unconiormablv 

 on all older rocks. In Miocene and Pliocene time the major volcanism 

 broke out and several large fields of nearly flat-lying units accumulated. 

 The Miocene and Pliocene volcanics generally rest unconformably on the 



