Approximately 3,000 Ar, Sr, and Pb isotopic age determinations for Canadian Cordilleran rocks have been cataloged, categorized as to reliability and significance, and plotted on histograms, distribution maps for different time intervals, and space-time plots to show the magmatic evolution in this 2,300-km portion of the Circum-Pacific Mobile Belt. The history revealed is episodic, with stable distribution patterns within episodes and distinct lulls and changes in distribution between the episodes. From 230 to 214 Ma (during Late Triassic time), extensive mafic volcanism occurred in the Wrangell, Quesnel, and Stikine terranes. Volcanic-related ultramafic complexes are found scattered through the two latter terranes. Large calc-alkaline granitic plutons are known only in a belt crossing Stikinia in northern British Columbia. At the same time, blueschists formed in the Cache Creek accretion wedge. From 214 to 200 Ma (end of Triassic and part of Early Jurassic time), Early to Middle Jurassic arc magmatism began in Wrangellia and in the northern Quesnel, Stikine, and Yukon terranes. A distinct magmatic event is recognizable only in southern Quesnellia. Magmatism was absent on the North American craton. The Cache Creek and Quesnel terranes were definitely linked, Stikine and Cache Creek terranes were probably linked, and a regional metamorphic episode was completed in the Yukon Terrane by this time. From 200 to 155 Ma (late Early to early Late Jurassic time), magmatism was extensive in the Wrangell, Quesnel, Stikine, and Yukon terranes. Magmatism over-lapped into North America only east of southern Quesnellia after about 180 Ma. By the middle of this time interval, the southern Quesnel-Slide Mountain-North America linkage was complete, and major deformation and metamorphism had affected the Omineca Belt in British Columbia. Early to Middle Jurassic magmatism in southern Wrangellia (Vancouver Island) is distinctly older than the Middle to Late Jurassic magmatism that occurred in central Wrangellia (Queen Charlotte Islands). From 155 to 140 Ma (during Late Jurassic time), a few last-gasp plutons of the late Early to early Late Jurassic episode and other rocks with partially reset 155- to 145-Ma dates occur in the Wrangell, Quesnel, and Stikine terranes. Late Jurassic magmatism (160 to 140 Ma) occurred in the Alexander Terrane (Saint Elias region). From 145 to 138 Ma (latest Jurassic and beginning of Early Cretaceous time), plutonism occurred in the Endako area of central British Columbia (Francois Lake suite) but is virtually unknown elsewhere. From 135 to 125 Ma (during Early Cretaceous time), there was a magmatic lull of major significance present throughout western North America. From 110 to 90 Ma (middle Cretaceous time), widespread plutonism occurred across all terranes. Dual culminations are evident: the Coast Plutonic and Ominica belts. Before this time all sutures except those outboard of Wrangellia had been closed. From 80 to 70 Ma (during Late Cretaceous time), a narrow, sinuous belt of magmatism persisted, mostly in the southeastern Coast Plutonic Belt, southwestern Yukon Territory, and scattered across the Skeena and Stikine arches. From 70 to 60 Ma (latest Cretaceous to Paleocene time), a distinct lull in magmatism occurred. Rare plutons of this time interval are known in the Coast Plutonic Belt, on the Skeena Arch, and in the southern Intermontane Belt. From 55 to 45 Ma (latest Paleocene to Middle Eocene time), widespread and voluminous magmatism occurred in all terranes. The early Cenozoic volcanic front crossed the Coast Plutonic Complex from its east side in the south to its west side in the north. Associated thermal and tectonic effects were strong even into the Omineca Belt, producing large reset metamorphic areas in the Coast and Omineca belts. This was a short-lived event, synchronous from southern British Columbia through the Yukon Territory. West of the volcanic front, offshore of Wrangellia, Metchosin volcano growth was underway at this time. Late in this time interval, the 50?–45–36-Ma Catface–Leech River event(s) of southern Wrangellia occurred. There is also time overlap with a diffuse Massett magmatic event in the Queen Charlotte Islands, and with Baranoff Island and Yakutat–Saint Elias region magmatism. Initial 87 Sr/ 86 Sr ratios and petrographic characteristics of Canadian Cordilleran igneous rocks are reviewed in the time frame just described. These reflect the nature of underlying crust, contemporaneous lithosphere thickness, and distance from the subduction zone. Comparisons with other parts of the Circum-Pacific Magmatic Belt shows both out-of-phase magmatism (Japan and southwestern Alaska) and perfect matching of some episodes (Sierra Nevada). Major magmatic episodes correspond to times of increased westward motion of North America with respect to hot spots or to times of increased convergence between western North America and the Farallon Plate.

Except for volcanoes in the Adel Mountains and near the Black Hills, the first 10 m.y. of the Cenozoic was a time of igneous quiescence. Basaltic volcanism in the eugeosyncline west of the present-day Cascade volcanic arc began in early Eocene time, was most intense between 54 and 44 m.y. ago, and tapered off slowly, with injection of basaltic and alkali trachyte dikes continuing until the Oligocene. In Oligocene time the eugeosynclinal rocks became welded to the continental margin, shorelines shifted westward, and volcanic activity west of the Cascade arc virtually ceased. Igneous activity began about 55 m.y. ago over a broad region in Washington, northern Idaho, and Montana, and during the early Eocene this activity swept southward across all the Northwestern United States. Volcanism, plutonism, regional high heat flow with associated geothermal circulation systems, block faulting, and ductile deformation at depth associated with tectonic denudation as a consequence of diapiric rise of plutonic rocks occurred synchronously during an intense culmination between 50 and 43 m.y. ago (Challis episode). Volcanism and deformation then died out abruptly as the locus of igneous eruption centers shifted south into Nevada-Utah and west into the Cascade arc, where volcanic activity persisted through the rest of the Cenozoic. Following widespread tectonic and igneous quiescence between about 38 and 18 m.y. ago, volcanic activity suddenly commenced over a large region with rapid eruption of Columbia River and related basalts. Volcanism, with bimodal chemistry, began during this episode (Columbia, 13 to 16 m.y. ago) in southwestern Idaho and over all of southeastern Oregon. Between 13 m.y. ago and today, bimodal igneous activity migrated eastward across Idaho to produce the Snake River Plain-Yellowstone volcanic field. In Oregon, at the same time, siliceous volcanic centers retreated westward. As a consequence rhyolitic volcanic centers today are most active only in Yellowstone and close to the Cascade Range. Striking synchroneity is shown by pulses of more intense igneous activity in the Cascade and Snake River Plain-Yellowstone regions.

Abstract A computer file of K-Ar, Rb-Sr, and U-Pb dates that provide constraints on the pre- Cenozoic Phanerozoic time scale has been created. New data appear slowly and thus the file size grows at the rate of only a few percent per year. The time scale presented at the 1974 International Meeting for Geochronology, Cosmochronology, and Isotope Geology in Paris is not in conflict with the data added since then. Precise chronologic subdivision of the Cretaceous is difficult because even the most optimistic uncertainties in the dates are greater than the duration of some stages. Nevertheless the stages of the Upper Cretaceous have been calibrated reasonably well. Subdivision of the remainder of the Mesozoic and Paleozoic systems cannot be done precisely and objectively from geochronometric data. Important boundary dates must be derived from interpolation between points with experimental and geologic uncertainties of at least a few percent. Efforts to obtain additional data for Lower Cretaceous to Upper Permian and Devonian and older rocks should be given special priority. One potential source of confusion in time-scale calibration, and conversely in the assignment of geologic age to rocks that have been dated, is the use of different decay constants by different laboratories and by the same laboratory at different times. Recently adopted values for uranium decay constants have the effect of reducing previously published U-Pb dates by about 1%. Proposed new decay constants for potassium (K) would increase previously published Phanerozoic K-Ar dates by about 2% in the case of western literature, and would reduce dates published by the eastern European countries by about 2.5%. The suggested new decay constant for rhubidium (Rb) is a compromise between values previously used. Most western and all eastern European Rb-Sr dates would be reduced by about 2%. Dates from western labs that have used the 47 x 10 9 -year half life would be increased about 3.5%. Revised time scales will reflect these changes. Because most calibration points are K-Ar dates the net effect is a 1 to 2% increase in the ages assigned to time-scale boundaries for scales published by geologists from western countries, and an approximate 2% reduction for scales published in eastern Europe. A discrepancy exists between time scales used in the two groups of countries. The eastern European scale is younger than the western one because of the greater use of glauconite K-Ar dates by the eastern Europeans. In contrast western emphasis is on dated volcanic rocks coupled with skepticism toward glauconite and whole-rock K-Ar dates.