TEMPORAL CHANGES IN THE SOURCES OF FLOOD BASALTS - ISOTOPIC AND TRACE-ELEMENT EVIDENCE FROM THE 1100-MA-OLD KEWEENAWAN MAMAINSE-POINT FORMATION, ONTARIO, CANADA

The Keweenawan Mamainse Point Formation, located on the eastern flank of a major 1100 Ma continental-rift/flood basalt province in the midcontinent of North America, consists of 5.3 km of continuously exposed picrite and basalt. The more than 350 lava flows exposed span nearly the entire duration of Keweenawan igneous activity and can be divided into eight groups on the basis of stratigraphic correlation of major, trace element, and neodymium isotopic compositions. Flows from the lower part of the section result from a mantle plume interacting with old subcontinental lithospheric mantle based on their epsilon(Nd)(1100) ranging from -5 to -0.5 and on their enriched incompatible trace element abundances with distinctive depletions in Ta and Nb. Flows from the middle of the section are dominated by crustal assimilation as evidenced by their depletions in P and Ti, enrichments in Ba, Th, La, and Ce, and low epsilon(Nd)(1100) of -9. Voluminous flow groups from the upper part of the section derive from a mantle source that is a mixture of plume and depleted mantle based on their consistently more radiogenic epsilon(Nd)(1100) of 0 to +3.5 and major and trace element compositions closest to the Proterozoic equivalent of MORB. A volumetrically minor flow group from the uppermost part of the section has very high incompatible element contents and epsilon(Nd)(1100) values between -1 and 0, showing renewed late-stage melting at low extents from the mantle plume. The progression of magmatic sources from plume+lithospheric mantle to plume+depleted mantle at this one locality parallels the magmatic evolution of other flood basalt provinces which are usually only developed over a much wider area.