Abstract

The late Cenozoic Mount Edziza Volcanic Complex covers an area of about 1,000 km2 in north-central British Columbia, 300 km east of the transcurrent boundary between the North American and Pacific plates. It is made up of a group of overlapping basaltic shields and intermediate to salic peralkaline composite domes, flows, and central volcanoes that are associated with extensional structures in the underlying basement. New K-Ar and fission-track dates (45) and Rb-Sr and Sr isotope analyses (12) from the Mount Edziza Volcanic Complex are reported. The age dates are for the most part consistent with the stratigraphy and indicate that frequent eruptive activity occurred during the past 8 m.y. Five major magmatic cycles each began with the eruption of basalt and culminated with the eruption of oversaturated peralkaline magma. Low 87Sr/86Sr initial ratios (0.7028 ± 0.0001) indicate a mantle source for the basalts. Low Sr contents and high 87Sr/86Sr ratios in the salic end members suggest that the oversaturated rocks were derived from the basaltic magma by crystal fractionation in crustal reservoirs. Rb-Sr isochrons suggest that residence times for the fractionating magma were about 0.7 to 1 m.y. Early removal of large amounts of plagioclase, followed by fractionation of potash feldspar, can account for most of the observed petrological and isotopic relationships. A few individual compositions and one suite of mainly intermediate samples contain anomalously large amounts of both 87Sr and radiogenic argon. This indicates that contamination with crustal material and possibly mixing of parental basalt with partly fractionated magma from previous events may have produced the relatively small volume of intermediate rocks.

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