Chemistry and origin of the Eocene Metchosin Volcanics, Vancouver Island, British Columbia

The Metchosin Volcanics of southern Vancouver Island consist of submarine pillow lavas, tuffs, and breccias, overlain by subaerial flows of mainly tholeiitic composition. They overlie basaltic dyke complexes and gabbro and are interpreted as Eocene (and older?) oceanic floor and oceanic island tholeiites. They are separated by the fundamental Leech River Fault from the Paleozoic and Mesozoic rocks underlying Vancouver Island. Major and trace element chemical analyses of the volcanics and a few related intrusive rocks are compared with those of recent to late Tertiary tholeiites from known oceanic settings and with the coeval Crescent Volcanics of the Olympic Peninsula. Possible settings are: (1) oceanic ridge (e.g., mid-Atlantic, Juan de Fuca); (2) oceanic ridge-island (e.g., Iceland); (3) oceanic intra-plate island (e.g., Hawaii); and (4) oceanic island arc (e.g., Tonga–Kermadec). Alkali–silica and AFM diagrams clearly define the subalkaline and tholeiitic character but do not allow distinction of these four oceanic settings. Major element factor analysis yields doubtful results but could perhaps be improved by increased data base. Several orthogonal plots of abundances of Ti, Fe, Mg, Zr, and Cr seem to permit distinction of the four types of tholeiites.On geological and chemical bases Metchosin and Crescent formations correspond most closely to the Icelandic ridge-island setting; the Hawaiian intra-plate island setting is less probable in view of several chemical distinctions. The ocean floor setting, though chemically similar in several plots, is precluded at least for the upper, nonmarine parts of the formations. Lastly, an island arc setting is not indicated by either general lithology or chemistry.