A paleomagnetic and U–Pb geochronology study of the western end of the Grenville dyke swarm: Rapid changes in paleomagnetic field direction at ca. 585 Ma related to polarity reversals?

• ∼585 Ma Grenville dykes show changes in primary magnetization direction of ∼90° in <4 My.
• They appear to record a paleo-equatorial field during reversals of the main dipole field.
• The changes may have occurred at a time when the Earth's field was frequently reversing.
• This unstable field behaviour predated the first Ediacaran fauna by only a few million years.
• Frequent reversals may help promote faunal evolution.

A paleomagnetic study of the western end of the ∼585 Ma Grenville dyke swarm shows that individual dykes are characterized by high coercivity and unblocking temperature magnetizations that can differ in direction by as much as 90°. Field tests including baked contact studies and the continuity of paleomagnetic direction along dyke strike, suggest that the magnetizations are primary. Precise U–Pb baddeleyite dating on these dykes indicates that changes in magnetization direction of ∼90° occur in less than 4 million years, and their close temporal association with reversals of the axial dipole field suggest that certain dykes are recording an equatorial dipole field as a transitional field between opposite polarity states. The documented instability in the Earth's field occurs less than 10 Myr before the first recorded appearance of macroscopic multi-cellular organisms on Earth, inviting speculation that an extended period of frequent magnetic reversals may play a part in faunal evolution.