Thick, layered peridotite–gabbro lava flows in Munro Township, Ontario

Abstract

Two unusually thick (120 m) lava flows in Munro Township, Ontario, are differentiated from peridotite at the base to gabbro near the top. One flow is komatiitic and contains the following units (from top to bottom): ultramafic flow-top breccia (19.5% MgO); olivine and pyroxene spinifex-textured rocks (17–10% MgO); gabbro (7% MgO); pyroxene cumulates (10–25% MgO); olivine cumulates (32–35% MgO); and a basal border zone. The composition of the lava at the time of eruption is believed to be represented by the ultramafic flow-top breccia. Following eruption, this lava differentiated to produce ultramafic cumulate rocks, and liquid of quartz-normative basaltic composition that solidified as gabbro.The other flow is tholeiitic and has a picritic hyaloclastite flow top (14% MgO) underlain successively by an aphanitic pyroxenite zone (but no spinifex texture), a gabbro zone, a thick clinopyroxene-rich cumulate unit, and a thin basal veneer of cumulate olivine.