Paleomagnetism of the Late Precambrian Coldwell Complex, Ontario, Canada

Abstract

The Coldwell Complex is a large multiphase alkaline intrusion on the north shore of Lake Superior. It was emplaced into Archean metavolcanics of the Superior Province in at least three distinct magmatic episodes during the Late Precambrian, giving isotopic ages ranging from 1010 to 1188 Ma. Detailed AF and thermal demagnetization was done on 416 specimens from 33 sites representing all petrologic phases. Additional data are included from an unpublished 1972 study by W.A. Robertson. The oldest magnetic component, PCA, was found in 19 episode I sites with a reversed mean direction of D = 121.0°, I = − 70.9° ( κ = 104, α 95 = 3.3°). Its pole position falls on the Keweenawan APWP at an age of 1109 ± 5 Ma. The second component, PCB, was found in 11 sites with a normal mean of D = 301.8°, I = 60.1° ( κ = 49, α 95 = 5.8°) that indicates an age of 1103 ± 5 Ma. The youngest component, PCC, was found in nine episode III sites and gives a reversed mean of D = 119.1°, I = − 54.3° ( κ = 101, α 95 = 5.1°) that indicates an age of 1095 ± 5 Ma. The assigned ages agree closely with recent U/Pb zircon ages but are distinctly older than the K/Ar and Rb/Sr ages. These paleomagnetic results require some changes in existing models for the petrologic evolution of the complex and indicate a much more rapid three-stage cooling history than previously postulated. They also provide strong evidence against the proposed concept of an asymmetrical geomagnetic field in Middle Keweenawan time.