Linking fabric and texture development to three-dimensional strain evolution in deformed quartzite and granitoid conglomerate clasts
Dyanna M. Czeck1, Eric Horsman2, Terra N. D. Anderson1 and Basil Tikoff3
1 Department of Geosciences, University of Wisconsin-Milwaukee, USA
2 Department of Geological Sciences, East Carolina University, USA
3 Department of Geoscience, University of Wisconsin-Madison, USA
Strain gradients in deformed conglomerates provide an opportunity to relate quantified strain to fabric and texture development. Using deformed conglomerates, we studied quartzite and granitoid clasts in detail to analyze their differing small-scale structural responses to increasing bulk finite strain.
We analyzed the Seine Metaconglomerates, which are found along the Wabigoon-Quetico Subprovince boundary within North America’s Superior Province and were deformed and metamorphosed to greenschist facies during granite-greenstone terrane development approximately 2.7 Ga. The metaconglomerates have a matrix of immature sand-clay size particles eroded from intermediate-mafic volcanic rocks and contain volcanic clasts with a range of felsic-mafic compositions, granitoid clasts largely of tonalitic composition, and quartzite clasts. Numerous structural features are consistent with a kinematic model of ductile transpression with oblique extrusion directions: subvertical foliations, a variety of lineation orientations, asymmetric dextral shear sense indicators on the subhorizontal plane, and primarily oblate strain shapes.
Strain analysis was conducted independently for each clast type using Rf/f analysis, bootstrapping statistics, and a least squares solution method to combine two-dimensional ellipses into three-dimensional ellipsoids. Octahedral shear strain (es) varied nonsystematically through the field area from 0.21-1.04 for granitoid clasts, 0.64-1.78 for felsic volcanic clasts, and 0.67-2.40 for mafic volcanic clasts. Unfortunately, quartzite clasts are too rare to produce quantitative strain measurements, but qualitative observations suggest that their strain magnitudes approximate strains measured for granitoid clasts. Based on these measurements and observations, within a given outcrop, clast competence hierarchy is quartziteŇgranite>>felsic volcanic>mafic volcanic.
Quartzite clast microstructures including undulose extinction and subgrain formation suggest deformation was accommodated primarily by dislocation creep. In granitoid clasts, quartz microstructures including undulose extinction and subgrain formation suggest it dominantly deformed by dislocation creep, whereas feldspar deformation was accommodated primarily by fracturing with minor dislocation creep. Within the granitoid clasts, the mica shape fabric intensifies and grain linkages increase during deformation through a combination of intracrystalline strain and dissolution-precipitation processes.
Quartz lattice preferred orientation (LPO) differed between granitoid and quartzite clasts at comparable strains. In the granitoid clasts, the quartz LPO intensity increases at low to moderate strain with progressive deformation, but plateaus at moderate to high strain. However, the quartz LPO intensity in quartzite clasts does not vary systematically with increasing strain, suggesting the existence in some cases of a strong primary LPO that is not obliterated by the observed strains. The feldspar LPO in the granitoid clasts increases at low to moderate strain with progressive deformation, but stabilizes at moderate to high strain. These results suggest the quartzite clasts, which likely were derived from a variety of sources, had acquired primary LPO textures prior to their incorporation into the conglomerate. The granitoid clasts have no evidence of primary LPO textures, and the observed LPO is likely a result of the metaconglomerate deformation. The relatively high strains accumulated in the deformation were not sufficient to reset the primary LPO present in the quartzite clasts.