QUARTZ MICROFABRICS AS INDICATORS OF HIGH- TO LOW-TEMPERATURE SHEARING UNDER VARIABLE RATES OF DEFORMATION

We present an investigation on microfabrics of quartz-porphyric veins from the Western Alps where various parameters of deformation and metamorphism can be determined and information about high strain-rate variations can be inferred. (1) Cooling rates of the wall rocks are known. (2) Based on deformation of euhedral magmatic quartz and on biotite orientation in the ground mass, strain can be determined. (3) The initial vein thickness can be calculated, ranging from ~ 1 to > 50 m. (4) Vein composition leads to the temperature of the magma, which – together with vein thickness and wall-rock temperature – allows to model cooling rates of the veins.

Since the magma was injected at T-peak of regional metamorphism, the quartz-porphyric veins cooled rapidly down to T ~ 450-600°C and then experienced slow cooling of regional metamorphism. A variety of high-T deformation fabrics developed in quartz, such as chessboard subgrain patterns, large recrystallized grains that partly grew up to the size of the magmatic host grain, and coarse grain-boundary sutures. Medium- to low-T fabrics include subgrains, dynamically recrystallized grains and small-scale grain-boundary suturing.

Brittle or semi-brittle deformation structures developed within this sequence of ductile structures and indicate variation of deformation rate: (1) fractures, along which arrays of recrystallized grains are formed; (2) kink-bands that are changed to arrays of subgrains and, by grain-boundary migration recrystallization, to arrays of dynamically recrystallized grains; (3) kink-band boundaries that are transformed to deeply serrated high-angle grain boundaries. In general, the detailed knowledge about various properties of the veins and their wall-rocks as well as the conditions of regional metamorphism and deformation allows far-reaching analyses of the interaction between rock fabrics and variable deformation rates.