Tracking fabric development with increasing finite strain in a deformed polymictic conglomerate

 

Dyanna M. Czeck, University of Wisconsin-Milwaukee, Milwaukee, WI, USA

Terra N. D. Anderson, University of Wisconsin-Milwaukee, Milwaukee, WI, USA

Eric Horsman, East Carolina University, Greenville, NC, USA

Basil Tikoff, University of Wisconsin-Madison, Madison, WI, USA

 

Clasts within deformed polymictic conglomerates can be used to quantify finite strain, and hence provide an opportunity to compare the response of clasts of different lithologies to deformation. In particular, we compare the deformation of two quartz-rich rocks, quartzite and granite, along strain gradients deformed at greenschist-facies conditions. The studied unit is the Seine Conglomerate, deformed within the wedge between the Seine River - Rainy Lake and Quetico shear zones in the Archean Superior Province in Ontario, Canada. Strain magnitudes, calculated using Rf/f method and bootstrap statistics, vary nonsystematically throughout the region. Strain is estimated by using the metavolcanic clasts, which have a similar lithology to the matrix: octahedral shear strain values (es) values range from 0.64 to 2.40 with a mean value of 1.29.

 

Quartzite clasts are generally less deformed than metavolcanic clasts, but there are too few quartzite clasts per outcrop to calculate a robust finite strain. The microstructures are consistent with deformation accommodated by dislocation creep. Quartzite clasts from strongly strained outcrops have a greater degree of recrystallization than clasts that have experienced weak bulk strain. Interpretation of LPO patterns in the quartzite clasts is not straightforward: LPO intensity does not necessarily correlate with increasing finite strain, and the LPO maxima do not match those expected from the field-measured foliation and lineation.  We interpret these results to be a residual signal of a previous LPO that is not obliterated by deformation of the metaconglomerate.

 

At any given outcrop, granitoid clasts have consistently smaller strain magnitude than metavolcanic clasts: es values range from 0.21 to 1:04 with a mean value of 0.51. Granite clasts are composed dominantly of quartz and albite, with lesser amounts of orthoclase and mica. Quartz and feldspar LPO intensity increases at low to moderate strain with progressive deformation, but plateaus at moderate to high strain. Quartz c-axes are sub-parallel to macroscopic lineation at low strain and form weak girdles subparallel to foliation at higher strain. In contrast, feldspar crystallographic axes maintain a stable orientation oblique to macroscopic fabric throughout deformation. Microstructural observations suggest dislocation creep was the dominant deformation mechanism in quartz, while dislocation creep and fracturing dominated for feldspar. Mica shows distinct changes with increasing finite strain. Mica shape preferred orientation intensifies and grain linkages increase during deformation though a combination of intracrystalline strain and dissolution-precipitation processes. Our results from the granitic clasts suggest that: 1) Quartz and mica were the dominant minerals accommodating strain throughout deformation; 2) The quartz and feldspar LPO development achieves steady-state at moderate strains.





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