Abstract

Quartz fabrics and microstructures in quartzites of the Mesnard and Goodrich formations were examined to better define the deformation and metamorphic history of the Marquette Synclinorium, northern Michigan. Fabric development is very weak in the whole-rock samples and indicates that mica contamination, extensive annealing, and (or) low imposed strains prevented the formation of distinct c-axis fabrics. However, well defined fabrics are preserved in five quartzite pebbles from the Goodrich conglomerate. Double maxima of varying intensities among these pebbles suggest that the northwest section of the district experienced inhomogeneous shear strain. Measured grain sizes yielded differential stresses ranging from 44 to 548 bar (4.4 to 54.8 MPa). Subgrain size data indicate stresses ranging from 151 to 248 bar (15.1 to 24.8 MPa). Dislocation densities determined by observed etch-pit densities using scanning electron microscopy indicate a range of stresses from 330 to 730 bar (33 to 73 MPa). Stress values from dislocation density measurements vary inversely with metamorphic intensity. Cumulatively, these microstructural data indicate that a low-stress regime of deformation preceded a cooler, higher stress pulse. The data also imply that deformation of the Marquette Synclinorium continued after peak metamorphism, contrary to early hypotheses.

You do not currently have access to this article.