Comparative geochemistry and petrology of Triassic basaltic rocks from the Taku terrane on the Chilkat Peninsula and Wrangellia

Upper Triassic metabasalt from the Chilkat Peninsula in southeastern Alaska is lithologically similar to the Middle and (or) Upper Triassic Nikolai Greenstone from the Wrangell Mountains, east-central Alaska. Both basaltic sequences show comparable petrologic and geochemical features. The informally designated Chilkat metabasalt, which forms part of a Triassic sequence between the Denali fault and the Chilkoot lineament, is included in the Taku terrane, whereas the Nikolai Greenstone characterizes the allochthonous Wrangellia terrane. The Chilkat metabasalt consists mostly of massive to inconspicuously layered flows with well developed pillow structures at the top of the section; the sequence may reach a total thickness of 3000 m. The metabasalt is predominantly amygdaloidal and contains secondary minerals typical of greenschist-facies metamorphism. The Nikolai Greenstone is a widespread, mostly subaerial but locally pillowed, amygdaloidal basaltic sequence that reaches a total thickness of more than 3500 m; it is commonly less metamorphosed than the Chilkat metabasalt and contains secondary minerals typical of zeolite-facies to very low greenschist-facies metamorphism. Both the Chilkat and Nikolai basaltic rocks are predominantly tholeiitic. The major-element compositions of both sequences are very similar and cluster within a narrow range. The trace-element data for both sequences suggest a composition similar to the high-magnesian Columbia River flood basalts. Sediments associated with the Chilkat and Nikolai basaltic rocks share gross lithologic features and a similar depositional environment, even though the upper part of the Chilkat sequence appears to be a more offshore facies. The petrologic and geochemical data are compatible with the inference that the Taku terrane may have been coextensive with Wrangellia during the Late Triassic but has subsequently been laterally offset by dextral movement on the Denali fault.