Radiometric dating, including numerous high-precision U-Pb baddeleyite and zircon ages, of Paleoproterozoic flood basalts, dike swarms, and layered mafic intrusions worldwide, indicates that a substantial volume of mafic magma was produced on Earth ca. 2.45 Ga. New U-Pb data presented here for the 2473 Ma Matachewan and 2446 Ma Hearst diabase dike swarms in North America establish a critical temporal link with ancient flood-basalt volcanism in Karelia and constrain the timing of a very ancient magnetic field reversal on Earth. The potential volume, magnitude, and extent of this magmatism may approach many of the better-known Mesozoic flood-basalt provinces and represent the oldest recognized large igneous province. The initiation and subsequent proliferation of flood-basalt and associated magmatism near the Archean-Proterozoic boundary may reflect a fundamental change in heat flux at the core-mantle boundary that heralded the breakup of a Late Archean supercontinent.

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