Quartz fabrics and quantified strains during transpressional deformation in the Seine Metaconglomerates

 

Anderson, Terra N., and Czeck, Dyanna M. Department of Geosciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201; tna@uwm.edu.

 

Many studies have quantified quartz fabrics in the field and in the lab. However, none of these studies have focused on deformation under natural conditions when detailed three-dimensional strain was quantified and when deformation mechanisms were evaluated. In our study, we link quartz shape and crystallographic fabrics within a deformed metaconglomerates across a strain gradient. Working with the calculated strains found by Fissler (2006) during strain analysis of the Seine Metaconglomerates from the Rainy Lake region in northwestern Ontario, we are utilizing petrographic and Electron Backscattered Diffraction (EBSD) techniques to document quartz grain fabrics.

Samples of oriented quartzite clasts and quartz veins were collected from several locations with different strain magnitudes within the Seine Metaconglomerates. The quartzite clasts have experienced the complete deformation history of the conglomerate while the syntectonic quartz veins only exhibit evidence of later deformation stages. Quartz is an ideal study subject due to its simple mineralogy and numerous deformation experiments.

Thin sections were made from quartzite and quartz vein samples with the x-direction parallel to lineation and the y-direction perpendicular to foliation. Petrographic analyses indicate that quartzite clasts and quartz veins deformed by dislocation creep with subgrain formation and recrystallization recovery mechanisms. Significant amounts of undulose extinction and some subgrains exist in all samples. Grain size and the amount of undulose extinction within grains decreased with increasing strain in quartzite clasts. Late stage fractures filled with calcite indicate a period of coeval semi-brittle features and fluid movement through the rocks during deformation that increased in size and abundance with higher strain. In the quartz vein samples, there is a strong shape preferred orientation subparallel to lineation of large quartz grains. With increasing strain, grain size decreases, undulose extinction becomes dominant only in the large remnant quartz grains, and the amount of calcite brought in by late fluids increases.

Using a SEM-EBSD system and CHANNEL5 software produced by HKL Technology, an automatic map of the thin section was used to determine the crystallographic orientations of quartz grains. The map was constructed to maximize the area analyzed on the thin section with one measurement taken from each grain.

Preliminary results of the EBSD analyses show increased clustering of quartz c-axes crystallographic preferred orientation (CPO) within quartzite clasts with increasing strain magnitudes. Similarly, in most cases, there was an increase in CPO of quartz c-axes within quartz veins when strain magnitudes increased. These observations taken with the microstructural observations are consistent with deformation by dislocation creep with recovery primarily by recrystallization.

One interesting feature that stands out when comparing the microstructural and EBSD studies is that the SPO of quartz grains decreases and the CPO of quartz grains increases with increasing strain. The increasing CPO without a corresponding increase in SPO is consistent with deformation by dislocation creep with recrystallization. We interpret the different microstructural observations between clasts of low to high strain to be consistent with dislocation creep that operated to a variety of strain magnitudes.


Return to publications.