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.

The massive 53.6 Ma Flat Creek granitic pluton of the Nisling Plutonic Suite intrudes flat-lying volcanic rocks of the 70 Ma Carmacks Group in the Stikine Terrane of the Intermontane Belt in the Yukon. Specimens (n = 334) from 22 sites in granite from the ~100 km 2 pluton plus 3 sites in cross-cutting 51.1 Ma Eocene andesitic dikes were tested using alternating field and thermal step demagnetization and magnetic susceptibility and saturation isothermal measurements. Magnetite was the sole important characteristic remanent magnetization (ChRM) carrier. Most granitic and andesitic specimens carried a lower temperature and coercivity normal-polarity A N ChRM and an antiparallel higher temperature and coercivity reversed A R component, but some specimens of both rock types carried just A R and some granitic specimens carried just A N components. Combining the A N and A R directions, the granite pluton yielded a mean direction of declination (D) = 165.2°, inclination (I) = −76.8°, (number of sites [N] = 31, radius of cone of 95% confidence [α 95 ] = 2.2°, precision parameter [k] = 139) and the dikes a mean D = 158.0°, I = −79.5°, (N = 3, α 95 = 6.0°, k = 423). Paleomagnetic contact tests proved inconclusive because of the contemporaneous and dual polarity remanence of the specimens. The pluton’s paleomagnetic pole indicates a nonsignificant northward displacement of 0.8° ± 4.6° and a significant clockwise rotation of 14° ± 10° for the Stikine Terrane relative to the North American craton after ca. 54 Ma. Regression analysis of ≤;54 Ma paleomagnetic motion estimates for all Intermontane terranes against time also shows nonsignificant translation with a significant rotation rate of 0.34° ± 0.11°/m.y. This implies that the Intermontane terranes since ca. 54 Ma have behaved as a quasi-coherent upper crustal plate that rotated atop a North American cratonic lower crust about a proximal near-vertical axis. It is speculated that the rotation was marked by westward extension in southern British Columbia and by eastward compression in the northern Cordillera, together amounting to ~550 ± 160 km of displacement orthogonal to the stable cratonic margin. The compression component in the north, driven by Pacific plate subduction and collision of the Yakutat terrane, is the suggested cause of arcuate orogenic uplift in the Mackenzie Mountains.