A continental volcanic arc of early Proterozoic age at Great Bear Lake, Northwest Territories

Hildebrand, Robert S. (1982) A continental volcanic arc of early Proterozoic age at Great Bear Lake, Northwest Territories. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The 1.875 Ga LaBine Group, which comprises mostly volcanic rocks, outcrops along the western margin of Wopmay Orogen at Great Bear Lake and rests on a deformed and metamorphosed 1.920 Ga sialic basement complex. It is overlain by rocks of the mainly rhyodacitic Sloan Group. Syn- to post-volcanic plutons of the Great Bear batholith intrude both groups. -- Facies relations and the overall evolution of the Group are closely comparable to Cenozoic volcanic fields believed related to subduction. Rocks of the LaBine field were hydrothermally altered by high-level geothermal processes but on the basis of SiO$\sb2$, TiO$\sb2$, REE, and phenocryst mineralogy they can be classified as calc-alkaline. Therefore, it is concluded that the LaBine Group represents an early Proterozoic volcanic arc developed upon continental crust. Laccoliths in Athapuscow Aulacogen together with recent geochronological and field data suggest that the LaBine Group postdates continent-microcontinent collision in Wopmay Orogen and was probably generated above an eastward-dipping Benioff zone which was either segmented or became shallower with time. -- Geochemical data and petrological considerations indicate that the magmatic rocks of the belt were generated by partial melting of lower continental crust and perhaps mixing of those magmas with slab-derived basaltic andesite. Preservation of high-level volcanic and plutonic rocks suggest that the region was never topographically high-standing. Therefore, the zone may be an early Proterozoic analog of the Longitudinal Depression of Chile and other intra-arc synclinal basins which seem to be the loci for major pyroclastic eruptions. The calculated volume of vitric ash removed from the basins by high-level atmospheric transport approximates the estimated volume of basaltic andesite intruded and extruded in island arcs. This may explain why the basins remain in isostatic equilibrium close to sea level. Because the volume of magma erupted and intruded in continental arcs is equal to, or exceeds, the volume of mafic magma rising out of the mantle, batholiths cannot be derived directly from the mantle--they are products of processes occurring in the continental crust.

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