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Alkalic igneous rocks of early Mesozoic age are found in both the Quesnel and Stikine terranes in the Canadian Cordillera and include both silica-undersaturated and silica-saturated types. The saturated complexes are most abundant in Quesnellia and are multiphase complexes dominated by monzonite to diorite intrusions. Undersaturated complexes are distributed through both terranes, are dominated by syenite with lesser monzonite and pyroxenite, and, when present as a single intrusion, are characterized by concentric zoning, igneous layering, and planar mineral fabrics. Both types of complex are associated with Cu-Au mineralization accompanied by potassic and distinctive sodic and calc-potassic alteration assemblages. Although undersaturated and saturated alkalic intrusions are petrographically distinct, a petrogenetic association is suggested by their spatial coincidence in some districts, and similarities in their tectonic environment and associated alteration. The undersaturated complexes represent a distinctive suite of alkalic intrusion with magmatic arc affinities, and their emplacement into both Stikinia and Quesnellia between 210 and 200 Ma suggests that these terranes were either linked at that time or have shared unusual but similar magma-generating tectonic events at identical times.

Distinctive regional Eocene metamorphic-plutonic units of the Coast Belt in southwestern Yukon match similar units in southeastern Alaska and British Columbia, and possibly in the eastern Alaska Range, and together form a belt originally >1200 km long. The final high-grade metamorphic event in the Aishihik and Kluane assemblages between the Ruby Range batholith and the Denali fault zone occurred in latest Paleocene to early Eocene time. Structural, petrologic, and geochronologic evidence indicates that metamorphism occurred during a major period of west-southwest-vergent deformation and metamorphism. The final phase of consumption of the Kula plate at approximately 43 Ma may be linked to the culmination of metamorphism, intrusion, and deformation along the early Cenozoic Cordilleran margin.

Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane, southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate Si[O.sub.2] contents, high Mg#'s (MgO/ (MgO+FeO*), [Al.sub.2][O.sub.3]/Ti[O.sub.2], and Zr(Hf)/middle rare earth element (REE) ratios; low Ti[O.sub.2], REE, and high-field-strength element contents; and U-shaped primitive mantle-normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks. Keywords: boninite, geochemistry, magmatism, continental crust, Yukon-Tanana terrane, arcs.

The end-Triassic marks one of the five biggest mass extinctions, but current geologic time scales are inadequate for understanding its dynamics. A tuff layer in marine sedimentary rocks encompassing the Triassic-Jurassic transition yielded a U-Pb zircon age of 199.6 [+ or -] 0.3 Ma. The dated level is immediately below a prominent change in radiolarian faunas and the last occurrence of conodonts. Additional recently obtained U-Pb ages integrated with ammonoid biochronology confirm that the Triassic Period ended ca. 200 Ma, several million years later than suggested by previous time scales. Published dating of continental sections suggests that the extinction peak of terrestrial plants and vertebrates occurred before 200.6 Ma. The end-Triassic biotic crisis on land therefore appears to have preceded that in the sea by at least several hundred thousand years. Keywords: mass extinction, U-Pb geochronology, biostratigraphy, Triassic-Jurassic boundary, Queen Charlotte Islands.