Geology and geochemistry of the East Bull Lake intrusion, District of Algoma, Ontario

Abstract

The East Bull Lake intrusion is a layered basic lopolith of early Aphebian age that intrudes Archean basement rocks at the southern margin of the Superior Province. It contains at least two magmatic cycles. Cumulate olivine, plagioclase, and clinopyroxene form anorthosite, troctolite, and olivine gabbro–norite in the lower portion of each cycle. These rocks are overlain by two pyroxene gabbros–norites in which orthopyroxene (or pigeonite) joins augite and plagioclase as the major cumulate minerals. Molecular proportion ratio (MPR) variation diagrams support this interpretation of the petrology of the intrusion and show that the major cumulate rock types represent magmatic liquids formed sequentially by fractionation of the above-mentioned cumulate phases.Small-scale phase layering is common in the intrusion. Size-graded layers are absent, and cross-bedded and igneous lamination features are not common. We conclude that this scale of layering formed in a boundary layer along the floor of the intrusion as a result of variable nucleation of the cumulate minerals. Much thicker isomodal layers of rock form the main mass of the intrusion. They may have formed by a double diffusion process that caused the formation of a density-stratified magma chamber.