Polybaric Fractionation in an Evolving Continental Rift: Evidence from the Keweenawan Mid-Continent Rift

Abstract

Evaluation of magma compositions in the Keweenawan Mid-Continent Rift shows that major element chemistry and variations in lava flow composition with stratigraphy were controlled by the physical evolution of the rift. In particular, evidence from the North Shore Volcanic Group of Minnesota reveals that primitive high-Al olivine tholeiite magmas saturated with olivine, clinopyroxene, and plagioclase at high pressure near the base of the crust were emplaced throughout the crust where they underwent lower pressure fractionation of olivine and plagioclase to again become triply saturated. As the rift evolved tectonically, magmas from successively deeper chambers were erupted until finally the deep-seated, voluminous primitive high-Al magmas were able to reach the surface, thereby explaining the predominance of high-Al basalt in the upper volcanic sequences and plutonic rocks.