QUANTITATIVE ANALYSIS OF FABRIC FORMATION LOCALIZED AT COMPETENCE CONTRAST ZONES BETWEEN BIOTITE/CHLORITE AND FELDSPARS IN THE SEINE RIVER METACONGLOMERATES, NORTHWESTERN ONTARIO, CANADA

The Seine River metaconglomerates are a deformed greenschist facies unit found at the boundary between the Wabigoon and Quetico subprovinces within the Superior Province of North America. The Seine displays a wide gradient of strain within the field region, which can be quantified using standard strain analysis techniques on conglomerate clasts. Within the Seine River metaconglomerate, as in many other deformed rocks, segregated bands of chlorite and biotite form the foliation. The quantifiable strain gradient ranging from very low to very high strain allows us to examine the genesis and evolution of foliation in naturally deformed rocks.

Our research is intended to investigate the fabrics resulting from a dramatic juxtaposition of competence contrasts between relatively “hard” minerals such as plagioclase feldspar and relatively “soft” minerals such as biotite and chlorite. At the microscopic level, biotite and/or chlorite grains appear to nucleate at the upper and lower boundaries of the competent grain, plagioclase feldspar. As deformation increases, the biotite and chlorite develop into continuous foliations continuing along the upper and lower margins of the feldspar. Using a quantitative approach, we examined the relative size and shape of competent domains and the resulting foliation bands. We documented the size of these biotite/chlorite foliations with respect to the size and shape of the competent minerals, and recorded their progressive development throughout the strained region. Preliminary results have shown a positive relationship between the length of the strand of biotite/chlorite and the size of feldspar grains at both low and moderate deformation. This suggests that with larger grain sizes, there is an increase in the magnitude of foliation nucleation. This ongoing research will help to clarify the role of mineralogical competence contrasts in the formation of deformation fabrics.