Stratigraphy and paleomagnetism of Mount Edziza volcanic complex, northwest British Columbia

Mount Edziza is the largest and most complex in a group of Late Tertiary and Quaternary volcanoes that lie along a north-south zone of nor mal faults along the easternside of the Coast Geanticline in northwestern British Columbia. It is a compo site shield and dome in which flat-lying lavas and pyroclastic rocks rest uncomfortably on tilted lavas and elastic sediments of t h e Early Tertiary Slok o and Sustutgroups. The products of Mount Edziza be long entirely to the alkali rock series and vary in composition from alkali o li vin e basalt through trachy basalt to peralkaline trachyte and sodic rhyolite. Atleast four principal magmatic cycles or stage s are recognized. Each stage began with eruption of primary basalt and culminated with eruption of a more acid phase. Rocks belonging to the earliest stages have been deeply dissected whereas the youngest flows and pyroclastic con e s show little or no evidence of erosion. Detailed paleomagnetic polarity pro fil e s of three strati graphic sections indicate that six reversals in the earth's magnetic field occurred during the life of the volcano. Correlation of the polarity events with radiometric ages and glacial stratigraphy indicates that volcanic activity began about 6 m .y.B.P ., during magnetozone 14, and continued intermittently throughout P le is tocene and into Recent time. During this long interval, successive centres of eruption we r e confined to a zone within a few mil e s of the present crater of Mount Edziza. Moreover, there is no evidence of progressive change in the character of the primary a lkalio livine basalt, or it's per alkaline fractionation products, that issued during the four successive stages of activity. Thus the volcanicity of Mount Edziza appears to reflect a consistent tectonic environment that has persisted throughout Late Tertiary and Quaternary time. The peralkaline nature of the Edziza lavas and the irassociation with north-south normal faults suggests a zone of crustal rifting.