PETROLOGY OF THE KEWEENAWAN MAMAINSE POINT LAVAS, ONTARIO - PETROGENESIS AND CONTINENTAL RIFT EVOLUTION

The Mamainse Point Formation forms a 5250-m-thick sequence of flood basalts, ranging from high-MgO picrites and basalts to basaltic andesites, that formed in the Keweenawan (1100 Ma) Midcontinent Rift of North America. This section is the most continuous and complete sequence of Keweenawan lavas and records almost the entire igneous history of the rift. The section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics. A variety of petrogenetic processes including partial melting of different sources at decreasing depth, periodically replenished, tapped, and fractionated magma chambers, crustal contamination, and polybaric fractionation were responsible for the evolution of the magmas. The dominant petrogenetic process changes with stratigraphy, which suggests that the physical evolution of the rift controlled which process operated at any time. The initial fracturing of the crust presumably allowed high-MgO liquids generated at depth to be erupted. The early stage of rifting was apparently slow and allowed the formation of long-lived magma chambers in which complex processes such replenishment/mixing and assimilation/contamination could occur. The late stage of rifting, however, was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted. These conclusions from the chemostratigraphy agree with the physical stratigraphy, which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation.