Subaqueous phreatomagmatic explosion breccias at Buttercup Hill, Noranda, Quebec

Abstract

Irregular, branching, white-fragment breccia dikes are restricted to the uppermost flows of the Amulet formation, where they occur within or are parallel to synvolcanic faults. At Buttercup Hill, the breccia dikes bifurcate upward and are continuous with a localized deposit of identical, conformable breccia within the lowermost andesitic flows of the Millenbach andesite formation. The dikes are shallow, poorly sorted intrusive breccias and like the overlying, conformable breccia deposit are characterized by white, angular fragments of variably silicified andesite derived from the immediately underlying Amulet formation and, to a lesser extent, from the Millenbach andesite formation. The fragments show a complete range in size from 0.5 m to < 1 cm and occur within a finer matrix that includes fine ash, broken feldspar crystals, and chloritized sideromelane shards. Sideromelane shards are foreign, juvenile fragments derived from underlying andesitic magma emplaced as dikes during the onset of volcanism.Breccias at Buttercup Hill were emplaced during successive, shallow, phreatomagmatic eruptions where rising andesitic magma contacted an external fluid. The coarse, conformable breccia at Buttercup Hill is similar to "explosion breccias," which constitute the basal deposits of subaerial maar volcanoes, which form during initial crater development. Buttercup Hill breccias and numerous other breccia dikes record a period of widespread hydrovolcanic eruption prior to and during the onset of andesitic magma resurgence that eventually infilled the Noranda cauldron.