THE GROWTH KINETICS IN SECOND PHASE AFFECTED SYSTEMS: THE DYNAMIC CASE

Deformation of a polymineralic rock, consisting of a matrix mineral and a subordinate quantity of second phases, requires interaction of different deformation and metamorphic processes, where the degree of interaction depends on a variety of extrinsic and intrinsic parameters. Of special interest in this context are processes related to interactions between matrix mineral and second phases responsible for the evolution of the microstructures. Studies on naturally deformed mylonites show that syndeformational grain growth of the second phases represents the major rate controlling process. At constant temperature, the grain size of the second phases (dp) increases with increasing volume fraction (fp). This tendency persists with increasing temperature but at constant fp the second phase size increases indicating thermally-activated and transport-controlled growth of the second phases. The second phase coarsening can be modeled by (1) dp=k exp(-Q/RT) fpⁿ, where k is a constant, Q the activation energy, T the temperature and n the growth exponent of the second phases. Since in polymineralic rocks the matrix grains are mostly affected by Zener pinning, syndeformational growth of the second phases controls directly the grain size of the matrix grain size (D): (2) D=cZ^m, where c is a constant and m the exponent of the Zener parameter (Z=dp/fp). The microstructural evolution of a mylonite during deformation can now be predicted by combining equations (1) and (2). Based on natural mylonites formed at different temperatures in a large-scale crustal shear zone we will demonstrate that second phase coarsening is enhanced in the shear zone and that Q, n, m, k and c can be directly obtained from the samples allowing the prediction of the microstructural evolution in polymineralic mylonites.