Syn-deformational grain growth: matrix coarsening during foliation development and regional metamorphism rather than by static annealing
Hickey, Ken A.; Bell, Tim H.
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1351 - 1374
published: Jan 8, 1997
manuscript accepted: Jul 19, 1996
manuscript received: Feb 16, 1996
ArtNo. ESP147050806014, Price: 29.00 €
Abstract The coarsening of stable mineral assemblages with increasing metamorphic grade, previously attributed to a reduction in the interfacial energy of an aggregate associated with periods of post-deformational annealing, occurs during foliation development by transfer of material down gradients in lattice strain energy. These gradients result from inhomogeneous deformation involving dislocation creep where the strain rate to temperature ratio e/T progressively decreases. Grain boundary migration and syntaxial overgrowth enable some grains to coarsen at the expense of others of the same phase. With a decrease in e/T during progressive regional metamorphism, a coarser maximum grain size can develop before the inevitable build up of internal strain energy results in a reduction in the size of individual grains by recrystallisation or dissolution. Syn-deformation metamorphic reactions aid coarsening of chemically stable phases by utilising them in a series of local catalytic sub-reactions that transiently increase intergranular gradients in free energy and promote greater redistribution of material within the rock. The devolatilisation associated with many reactions enhances redistribution by temporarily increasing the fluid content and removing graphite adsorbed onto crystal surfaces. Consumption of matrix phases during reaction promotes coarsening by reducing the number of interphase grain boundaries. Gradients in interfacial energy are smaller than those produced by strain energy during dislocation creep. Consequently, they are unlikely to control the transfer of material for coarsening for all but the finest grained aggregates that have been subjected to low strain rates and high temperatures. Interfacial energy driven growth may occur after periods of deformation but be relatively insignificant, because the higher gradients in free energy and increased rates of fluid circulation associated with inhomogeneous deformation strongly favour syn-tectonic growth.