New volcanological and geochemical observations from the Blake River Group, Abitibi Greenstone Belt, Ontario and Quebec: Tannahill Township and Lake Labyrinth area

Abstract

The Archean Blake River Group of Ontario and Quebec is dominated by submarine mafi c to intermediate volcanic rocks, with more restricted felsic volcanic rocks. The monotony of the mafi c to intermediate volcanic sequences is broken by the occurrence of basaltic to andesitic volcaniclastic rocks. These range from very localized pillow breccia and other small-volume lava-related fragmental rocks, to mapped or inferred kilometre-scale volcaniclastic units. Remapping of most of these volcaniclastic units demonstrates that in some cases their importance had been exaggerated. Furthermore, volcaniclastic rocks having different characteristics and origins had been agglomerated together (sometimes even with signifi cant coherent volcanic rocks). A case in point is the sector around Lake Labyrinth, where volcaniclastic rocks include monomictic rocks (pillow breccia, in situ volcanic breccia, hyaloclastite) and polymictic rocks (including felsic clasts). One of the largest 'volcaniclastic unit' on former maps actually consists of coherent mafi c volcanic rocks, coherent felsic rocks, and volcaniclastic rocks. The last are variable in appearance, ranging from apparently monomictic to apparently polymictic, from aphyric to plagioclase-phyric, and from mafi c to felsic. Farther north in Tannahill Township, volcaniclastic rocks are also more restricted than suggested by some workers. In the southeastern corner of the township, the monomictic volcaniclastic rocks show rare jigsaw-fi t textures and chilled margins, and are interpreted overall as remobilized hyaloclastite. These rocks are chemically similar to the lavas from the rest of the southern part of the township and are inferred to be locally derived and emplaced on the fl anks of a submarine volcano.