An unusual cleavage is developed in troctolitic rocks of the late Precambrian Duluth Complex in northeastern Minnesota. It is localized in a zone −250 m wide, and it is parallel to the axes of gentle folds in rhythmic banding. The rocks show no textural signs of ductile deformation, and the cleavage is defined by fractures that are fairly straight and evenly spaced, but on a fine scale, somewhat irregular, multiple, anastomosing, and locally crosscutting. Textures indicate a purely extensional mode of development. Cleavage refraction across interfaces of bands of different composition is common. Serpentine coats the fracture surfaces and suggests that fluids were present to produce this alteration during formation of cleavage.

The geological relationships and serpentinization suggest conditions of cleavage formation of 1.5–1.8 kb and 350 to 500 °C. The presence of extensional fractures indicates that cleavage formation involved crack propagation at high fluid pressures. The absence of shear fractures or ductile deformation features, and the nature of the cleavage refraction indicate that differential stresses were low.

Localization of the cleavage seems to be due to the presence of a structure, probably a dike, below the layered intrusive rocks. This could have served to localize stresses and reactive fluids at high pressure during a late stage of continental rifting.

First Page Preview

First page PDF preview
You do not currently have access to this article.