AGU Fall Meeting.
Paper T33C-1506

Texture development in naturally deformed granite, Seine metaconglomerate, Ontario

Horsman, E
Dept. of Geology & Geophysics, Univ. of Wisconsin - Madison, 1215 W Dayton St, Madison, WI 53706, United States

Czeck, D
Dept. of Geosciences, Univ. of Wisconsin - Milwaukee, PO Box 413, Milwaukee, WI 53201, United States

Tikoff, B
Dept. of Geology & Geophysics, Univ. of Wisconsin - Madison, 1215 W Dayton St, Madison, WI 53706, United States

We present lattice-preferred orientation (LPO) data and microstructural observations from granite conglomerate clasts deformed at greenschist facies conditions in the Archean Seine River - Rainy Lake shear zone region, Ontario, Canada. Samples for analysis were selected from a strain gradient recognized with detailed three- dimensional strain analysis, allowing us to interpret our results in the context of progressive deformation. Granite clasts are composed dominantly of quartz and albite, with lesser amounts of orthoclase and mica. Mica abundance increases slightly during deformation as feldspar reacts with metamorphic fluids. Mica shape- preferred orientation intensifies and grain linkages increase during deformation though a combination of intracrystalline strain and dissolution-precipitation processes. Quartz LPO intensity increases at low to moderate strain with progressive deformation, but plateaus at moderate to high strain. Microstructural observations including undulose extinction and subgrain formation indicate that dislocation creep was the dominant deformation mechanism at low strain. At higher strains, the prevalence of smaller, undistorted grains indicates that recrystallization was an important recovery mechanism. Feldspar LPO undergoes similar intensity variation from low to high strain, but the magnitude of variation is less than that for quartz. Microstructures in feldspar grains include through-going fractures and undulose extinction at all magnitudes of strain. We infer both dislocation creep and fracturing deformation mechanisms for feldspar. Quartz c-axes are sub-parallel to macroscopic lineation at low strain and form weak girdles sub-parallel to foliation at higher strain. Asymmetry of the quartz LPO is consistent with dextral shear in the region. In contrast, feldspar crystallographic axes maintain a stable orientation oblique to macroscopic fabric throughout deformation. Our results suggest that quartz and mica were the dominant minerals accommodating strain throughout deformation. Feldspar was not entirely rigid, but accommodated little strain. The consistency of LPO observed in both quartz and feldspar at moderate to high strain demonstrates that intensity does not continually increase. Rather, some minerals may develop a steady-state LPO in granite deformed at greenschist facies conditions.

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