THE NATURE OF VOLCANO-PLUTONIC RELATIONS IN THE GREAT BEAR MAGMATIC ZONE, NORTHWESTERN CANADA

The connection between the volcanism and plutonism has been of interest to geologists since they realized that plutons were once low viscosity magma. Despite numerous studies involving thousands of chemical and isotopic analyses the connection has proven difficult to resolve and is still obscure, largely because there are few places that have enough relief to expose the critical relations. The Great Bear magmatic zone, a Paleoproterozoic continental arc located in northern Canada's Wopmay orogen, provides an informative field setting to resolve some of these issues because the rocks are generally non-metamorphosed and were broadly folded such that calderas, stratovolcanoes, and a wide variety of plutons are exposed in oblique cross-section on limbs of shallowly-plunging folds in an area of subdued topographic relief.

Early mafic plutons intruded co-magmatic pillow basalt piles as thin sheets with aspect ratios of 10-15. Plutons of intermediate composition, temporally and spatially associated with porphyritic andesitic stratocones, have flat or slightly domical roofs and flat floors, zoned alteration haloes, and have aspect ratios in the range of 5-10. Dominantly granodioritic to monzogranitic plutons that cut thick sequences of ignimbrite are generally sheet-like bodies with aspect ratios of 10-20, except where they intrude calderas and form resurgent plutons. Many, if not all, of the ignimbrite sheets are compositionally zoned, but the plutons, despite compositional variations nearly as large as the zone as a whole, are generally not zoned in an obviously systematic manner.

After each eruption type, magmas of similar composition and texture rose into the volcanic suprastructure to form sheet-like plutons. The plutons, which appear broadly comagmatic with their wallrocks, cannot have fed them because they cut them and are thus demonstrably younger. We suggest that following eruption and partial evacuation of compositionally zoned magma chambers, the chambers were re-energized, perhaps by influx of additional magma, and rose into their own eruptive products. The cycle of eruption, with the partial evacuation of chambers and subsequent rise of remaining magma to even higher levels in the crust explains why it is generally so difficult to link volcanic eruptions to specific plutons.