HR: 1330h
AN: T42A-0269    [PDF]
TI: The Influence of Second Phases on Grain Boundaries of Mylonitic Microfabrics: Evidences From Natural Carbonate Mylonites
AU: Ebert, A
EM: ebert@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstr. 1-3, Bern, 3012 Switzerland
AU: * Herwegh, M
EM: herwegh@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstr. 1-3, Bern, 3012 Switzerland
AU: Pfiffner, A
EM: pfiffner@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstr. 1-3, Bern, 3012 Switzerland
AB: Impure carbonate mylonites were sampled along basal thrusts of two Helvetic nappes in western Switzerland (Morcles and Doldenhorn nappe). The two nappes built up large-scale recumbent folds with intensively deformed and elongated inverted limbs. In both nappes the temperature rises towards the rear of each tectonic unit. Both type, mineralogy (sheet silicates, dolomite, quartz) and content of second phases vary within the samples. In this study we investigated the influence of volume fraction, grain size, shape, mineralogy of second phase minerals, and the temperature on the mylonitic microfabric. In all samples, calcite shows a steady state grain size, but the grain size varies as a function of (a) size and volume fraction of the second phase minerals and (b) temperature. (a) At constant temperature, two major microstructure controlling parameters can be distinguished: second phases in impure and dynamic recrystallization in pure mylonites. In impure calcite mylonites pinning and dragging of calcite grain boundaries by second phase minerals stabilize the calcite grain size (D$_{cc}$) after: D$_{cc}$=c* Z$^{m}$ with Z=d$_{p}$/f$_{p}$ (size d$_{p}$ and volume fraction f$_{p}$ of second phase minerals, modified after Zener in Smith, 1948). In this way the calcite grain size is inversely proportional to the second phase content. In contrast, the average calcite grain size is maintained constant by dynamic recrystallization via a balance of grain growth and grain size reducing mechanisms. Different lithologies with different second phase mineralogy from the same locality (constant temperature) show the same D$_{cc}$-Z trends, indicating that the shape of the second phases (elongated sheet silicates, blocky dolomite, spherical quartz) has no major influence on the calcite grain size. However, the elongation of calcite grains increases with decreasing Z, while in pure samples the elongation is constant. (b) With increasing temperature the steady state grain size D$_{cc}$ of calcite increases, while the aforementioned D$_{cc}$-Z trends persist. Thus for both types of microstructure c is temperature dependent. In light of crystallographic preferred orientation, first investigations show a shift from random distribution to point maxima with increasing Z and temperature. This study shows that already small amounts of second phases can drastically influence the microstructure of mylonites and the associated deformation mechanisms. The aforementioned relation allows to take the influence of second phases on calcite grain size into account, which is an important requirement for rheological interpretations.
DE: 3660 Metamorphic petrology
DE: 3947 Surfaces and interfaces
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2003 Fall Meeting