The role of quartz during deformation of polyphase rocks

In this thesis, deformation mechanisms, rheology and the related microstructural evolution of quartz during deformation of polyphase rocks are investigated in natural, high temperature, small scale shear zones in granitoids. Selected shear zones are from the Gran Paradiso Nappe, Western Alps, Italy, and the Truzzo granite of the Tambo Nappe, Central Alps, Italy. The microstructural evolution inside strain gradients from non- or weakly deformed hostrocks to ultramylonitic shear zones is studied by optical microscopy, scanning electron microscopy and image analysis, crystallographic preferred orientations (CPO) by means of orientation imaging using computer integrated polarization microscopy (CIP) and electron backscatter diffraction (EBSD).
Magmatic quartz grains recrystallize dynamically, and form polycrystalline aggregates that deform by dislocation creep inside a fine grained feldspar-mica matrix, which deforms by diffusion creep. With increasing strain, quartz aggregates form layers and finally disintegrate into a grain-scale polymineralic mixture with K-feldspar, plagioclase and mica. The disintegration of quartz aggregates marks the transition from a mylonite to an ultramylonite and occurs by intergranular dilatancy related to grain boundary sliding and by the precipitation of K-feldspar and biotite.
Polycrystalline quartz aggregates deform as porphyroclasts embedded in a lower viscous matrix at low and constant differential stresses. This situation is approximated by Reuss-bound conditions. Quartz aggregates disintegrate because quartz fails to deform by dislocation creep compliant with the matrix.
In the Gran Paradiso shear zones (lower amphibolite facies, ~500-550°C), a stable quartz grain size forms by a dynamic equilibrium. Subgrain rotation recrystallization and grain boundary migration recrystallization (synkinematic grain growth) contribute to a grain size decrease and increase respectively. In the ultramylonite the quartz grain size decreases below the quartz subgrain size and approaches the matrix grains size. The dynamic quartz grain size is decreased by a combination of pinning and dissolution processes. The constant quartz volume fraction requires quartz precipitation. Pinning eliminates the contribution of synkinematic grain growth, and dissolution permits a quartz grain size below the subgrain size. The ultramylonite deforms by dissolution-precipitation assisted diffusion creep. Quartz in the ultramylonite shows isotropic grain shapes and a random CPO.
Polycrystalline quartz aggregates in the Gran Paradiso mylonites develop a strong CPO consistent with the activity of the basal-<a> slip system. The CPO of each aggregate develops with respect to a local kinematic framework and not to the shear zone reference frame. The local reference frame is defined by the quartz aggregate and its position in the matrix. A strong CPO develops already at low strain, and peripheral [c]-axis maxima reach a stable position at about 70° with respect to the flow plane.
Quartz aggregates show a local shear sense, which, at low strain is systematically opposite to the global shear sense. The inverse shear sense is interpreted to result from flow partitioning between the higher viscous quartz aggregate and the lower viscous matrix in bulk simple shear.
The quartz fabric is in most cases related to the CPO such that the maximum of the surface orientation distribution function is synthetically rotated with respect to the sense of slip on the quartz basal plane. This situation implies a crystallographic control of the fabric development. Orthorhombic surface fabrics are suggested to form at high grain boundary mobility, monoclinic surface fabrics from at lower grain boundary mobility.
In the Truzzo granite shear zones (amphibolite facies, ~550-650°C), dynamic recrystallization of quartz is dominated by grain boundary migration with an increasing contribution of subgrain rotation related microstructures during advanced stages of deformation. The ultramylonite part of the shear zones deforms by diffusion creep. Single quartz grains show a shape anisotropy and a very weak CPO, interpreted to result from a contribution of intracrystalline plasticity.
Local biotite breakdown and subgrain rotation - bulging recrystallization occurs in thin, newly coalesced layers which represent the very latest structures in the ultramylonite. This relates to the inversion of the viscosity “contrast” between quartz and the feldspathic material at higher differential stresses and lower temperature.
Theoretical and experimentally derived flow laws from literature are tested with the data obtained in this thesis. The quartz - feldspathic behavior can be simulated, resulting in geologically reasonable strain rates and viscosity ratios.
In the Truzzo granite shear zones, Fourier transform infrared spectroscopy (FTIR) measurements inside single grains reveal that quartz remains dry during recrystallization at the main deformation event. FTIR spectra are flat and the water content is comparable to that of brazil quartz. Fluid inclusions are expelled during grain boundary migration recrystallization. Deformation took place at water present conditions, and quartz grain size piezometers suggest low differential stresses while dry quartz is considered extremely strong during experimental deformation. It is suggested that grain boundary processes contribute to the commonly observed weakening of polycrystalline quartz during fluid present conditions and that the low water concentrations might be sufficient for crystal plasticity at natural conditions.