The nature of volcano-plutonic relations and the shapes of epizonal plutons of continental arcs as revealed in the Great Bear magmatic zone, northwestern Canada

The shapes of plutons and their emplacement mechanisms, the connection between the volcanic and plutonic realms, and the development of batholiths have been of interest to geologists since they realized that plutons were once low viscosity magma. These issues have proven difficult to resolve 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 fold limbs 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 associated with andesitic stratocones, have flat or slightly domical roofs and flat floors, and aspect ratios in the range of 5-10. Granodioritic to monzodioritic plutons that cut thick sequences of ash-flow tuff and related volcaniclastic rocks are generally sheet-like bodies with aspect ratios of 10-20, except where they intrude calderas and form resurgent plutons. Granitic plutons intrude at slightly deeper crustal levels, are generally younger, and typically have miarolytic cavities, pegmatites, and associated dike swarms. The granites have flat roofs and floors but generally have lower aspect ratios than the intermediate composition plutons. Cycles-where magma bodies were first partially evacuated by eruption, then were re-energized and rose into their own ejecta to form plutons-span the compositional range from basalt to rhyodacite. 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. The overall development of the Great Bear magmatic zone-from small-scale local eruptions of basalt to voluminous eruptions of intermediate composition ash-flow tuff followed by wide-scale emplacement of granitic plutons-is interpreted to represent input of subduction-related magmas, which led to progressive heating, melting, and wholesale upward differentiation of the crust beneath the arc.