The Wrangell lavas in the St. Clare province of southwestern Yukon are part of the larger Wrangell volcanic belt that has been active throughout the Late Cenozoic. These lavas have erupted in a transitional tectonic environment that reflects regional transpression along the Queen Charlotte transform-Fairweather-Totschunda Fault System and subduction of the Farallon Plate beneath North America. The volcanic province is composed of subalkaline basalt (31%), basaltic andesite (30%), andesite (21%), dacite (2%) and nepheline normative basalt (16%).
The hypersthene normative basalt is (in order of appearance) spinel-olivine-plagioclase ± Fe-Ti oxide ± clinopyroxene phyric, whereas andesite contains plagioclase, Fe-Ti oxide, clinopyroxene, ± orthopyroxene phenocrysts, and dacite and intrusive latite contain phenocrysts of plagioclase, ± clinopyroxene, hornblende, ± biotite, ± sanidine.
The nepheline normative rocks, where porphyritic, contain phenocrysts of olivine, plagioclase and hornblende. In the central part of the map area, the lowermost flows are nepheline normative basalt that are interbeddded with clastic sediments and overlain by basaltic andesite, andesite and volcanic conglomerate. This succession is overlain by basalt interbedded with clastic sedimentary rocks and pyroclastic rocks. In the southern part of the map area, alkaline basalt occurs at this stratigraphic level. The uppermost Wrangell lavas are andesitic with minor interbedded volcaniclastic rocks.
The hypersthene normative lavas of the St. Clare province are transitional in terms of their Na2O + K2O/SiO2 ratios between alkaline and subalkaline magma series and in terms of their FeO/MgO versus SiO2 ratios between tholeiitic and calc-alkaline series. Chemical composition of these rocks reflects the unique tectonic setting within which they are found.