Interpretation of the relations between massive, pillowed and brecciated facies in an archean submarine andesite volcano — amulet andesite, rouyn-noranda, Canada

Abstract

The Amulet andesite formation in the Archean terrain of Rouyn-Noranda, P.Q., consists of 19 flows, distinguished by variations in phenocryst content and vesicularity and by the presence of concentric contraction fractures. Detailed mapping of flows revealed the presence of three main facies: (1) massive facies; (2) pillowed facies; and (3) foreset-bedded (brecciated) facies. The massive facies consists of > 50% massive lava overlain by pillow lava and/or pillow breccia. The pillowed facies consists of > 50% pillows. In some flows, the pillowed facies contains a thin sheet of massive lava at the base of the flow (facies 2a). Generally, massive lava fills braided channels 10–50 m wide (facies 2b). Facies 2c consists of pillows and large, irregular, megapillows. Determination of the flow direction shows that facies 1 is proximal, facies 2 distal. The foreset-bedded facies 3 consists of alternating thin (1–2 m) lobes of massive lava, pillow lava and broken-pillow breccia. It makes up flows 16–19 at the top of the sequence. Flow M, a unit entirely composed of massive lava, is ponded against flows 9 and 10. We interpret the growth of the Amulet andesite volcano in the light of new work on ocean-floor basalt and on the 1969–1973 Mauna Ulu eruption. The feeding fissure was located in the area of massive facies. At the beginning of eruptions lava spread laterally as a sheet-flood flow (massive base of facies 2a) but rapidly became channelized. The pillow lava and in particular the facies 2b, 2c and 3 are considered as the subaqueous equivalents of tube-fed pahoehoe. Flow M probably represents a lava lake. Shallowing-upward of the sequence during the built-up of the volcano is indicated by increasing vesicularity of the flows and by an upward increase of the proportion of broken pillow breccia. This increase is gradual from flow 1 to flow 15 but abrupt from flow 15 to 16–19.