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; firstname.lastname@example.org.
Previous studies have documented quartz fabrics in the field (e.g. Bestmann et al., 2004; Lisle, 1985; Stipp et al., 2002) and the lab (e.g. Heilbronner and Tullis, 2002; Hirth and Tullis, 1992; Mainprice and Paterson, 1984). However, these previous studies have not been conducted in areas with deformation under natural conditions where strain can be quantified and deformation mechanisms have been determined. In our study, we link quartz shape and crystallographic fabrics within a deformed metaconglomerate across a strain gradient. Fissler (2006) conducted strain analysis on the Seine Metaconglomerates from the Rainy Lake region in northwestern Ontario. She determined strain independently for various lithological groupings of clasts and determined various degrees of strain types and magnitudes throughout the region. Using her strain results as a framework for our fabric analysis, we use petrographic and Electron Backscattered Diffraction (EBSD) techniques to document the quartz grain fabrics throughout the region.
Fissler (2006) classified twenty-six outcrops of the Seine Metaconglomerates as low, medium, or high strain. Oriented quartzite clasts and quartz veins were collected from several locations with different strain magnitudes. The quartzite clasts experienced the complete deformation history of the conglomerate whereas the quartz veins, having been intruded during the deformation, only exhibit evidence of later deformation stages. The quartzite clasts and quartz veins have a simple mineralogy that is readily compared to numerous results from 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. 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. The step size was determined by the average grain size and ranged from 150 to 200 microns. The data was entered into PFch5, a pole figure-plotting program developed by Mainprice (2005).
Microstructures in the Seine have been studied in detail by Czeck (2001) and Fissler (2006). Both found evidence for very similar deformation mechanisms at all strain magnitudes in a range of clast types. Rocks were primarily deformed by diffusive mass transfer processes with some dislocation and micro-fracturing processes in certain phases. Using a petrographic microscope, we conducted additional microstructural analyses to infer deformation mechanisms within the quartz samples because the previous studies did not focus on quartz.
Petrographic analyses indicate the following microstructures. In low-moderate strained quartzite clasts, grain size is bimodal with larger grains 20 to 40 mm and smaller grains around 5 mm in diameter. Grain boundaries are irregular and show slight preferred orientation parallel to lineation, forming a shape preferred orientation (SPO). Within the grains, undulose extinction is common and some subgrains exist. Some recrystallized grains, primarily as ³necklaces² around the larger quartz grains, can be observed at the edges of larger quartz grains. We interpret that the quartz underwent deformation primarily by dislocation creep with subgrain formation and recrystallization recovery mechanisms. Late stage fractures occurred which cut the main fabric and allowed fluid movement resulting in veins filled with calcite and some minor amounts of mica.
In highly strained quartzite clasts, grain size is uniform ranging from 5 to 10 mm. The round, equant grains have regular smooth boundaries that often intersect at triple junctions. Internally, the grains often have undulose extinction throughout, and some subgrains are observed. Calcite fills in fractures that are oblique to lineation. The grain size near these zones is smaller, less than 5 mm in diameter. We interpret that the quartz grains are deforming primarily by dislocation creep, and that the reduced grain size is due to extensive recrystallization. The number and size of late stage fractures increased from the low strained examples. The fractures cut the main fabric and filled with calcite and more mica relative to the low strain clasts.
In low-moderate strained quartz veins, grain sizes vary greatly from 20 to 500 mm. Grain boundaries are irregular and show a slight SPO subparallel to lineation. Some ~5 mm recrystallized grains are observed at the edges of the larger quartz grains. These are seen primarily as ³necklaces² around the larger quartz grain edges.
In moderate-highly strained quartz veins, grain size is fairly uniform ranging from 20-40 mm in diameter. Some larger grains that are 500 mm in diameter exist, often with ³necklace² recrystallization of quartz and calcite around them. Boundaries are irregular and larger grains have undulose extinction. Calcite fills in fractures that are oblique to lineation.
Preliminary results of the EBSD analyses show increase cluster of crystallographic preferred orientation (CPO) in the c-axis of quartz 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 are consistent with deformation by dislocation creep with recovery primarily by recystallization.
One interesting feature that stands out when comparing the microstructural and EBSD studies is that the SPO of quartz grains decreases with increasing strain, 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.
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