Magnetization of the Spanish Peaks Dike Swarm, Colorado, and Shiprock Dike, New Mexico

A detailed study of the magnetic properties of some of the radiating dikes from the Spanish Peaks, Colorado, and from a pair of dikes from Shiprock, New Mexico, indicates that there is no correlation between the direction of stable remanent magnetization and dike orientation. Some dikes have a stable component of remanent magnetization; others do not. In general, magnetic stability is correlated with the presence of small grains observed microscopically. In most samples ilmenite lamellas are few or entirely lacking, indicating that high-temperature oxidation has been minimal. However, most samples have been affected to some degree by low-temperature oxidation resulting in the alteration of titanomagnetite to titanomaghemite. There is no relation between the presence of maghemite and the degree of magnetic stability. The dikes with higher magnetic stability were used to determine a pole position for the Eocene of North America. This position is 81°N, 149°W, and is close to other lower Tertiary pole positions.