Subaerial volcanism in the Archean Uchi-Confederation volcanic belt

Abstract

The Uchi-Confederation Lakes area, within the Uchi subprovince of the Canadian Shield has three major cycles of mafic to felsic metavolcanic rocks forming a section 8500-11,240 m thick, folded about a central syncline. A rhyolite tuff about 300 m thick is exposed over a strike length of 16 km at the top of the second cycle. The tuff is eutaxitic in texture and consists of relict shards, fiamme and essential pumice and lithic fragments. The unit forms two flow units and one cooling unit approximately 300 m thick, with a lateral extent of 12 km, consisting of a vitrophyric base, a densely welded mid-portion about 200 m thick and the upper 40 m which is less densely welded. The fiamme, up to 38 cm long and 5 cm thick, are draped over lithic fragments and wholly replaced by very fine-grained quartz. Snowflake texture is observed in feldspar grains in the relict glassy part of the unit. Pronounced reverse size grading is present with pumiceous and lithic fragments concentrated in the upper part. The upper portion contains breccia-filled fractures indicative of fumarolic activity. The unit is underlain by subaqueous ash flows and overlain by stromatolitic carbonate. Reconstruction based upon poor stratification, reverse size grading, abundant relatively fine pumiceous clasts and great lateral extent suggest a Plinian eruption and caldera collapse. The unit represents subaerial deposition and welding based on replacement of fiamme by silica, development of snowflake texture, and deformation of fiamme and welding of shards. Submergence and deposition of shallow water sedimentary carbonate followed.