Deformation mechanisms and reaction of hornblende: examples from the Bergell tonalite (Central Alps)

Abstract

Hornblendes in the Bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization. During the crystallization of the magma deformation commences by magmatic flow. In this stage the hornblendes behave as rigid particles in a viscous matrix (melt). The rotation and alignment of hornblende as elongate, rigid particles have produced a strong preferred orientation of both, particle long axes and crystallographic directions. With progressive solidification of the melt, there is a gradual transition from magmatic flow to solid-state deformation in the Bergell tonalite. The crystallographic and shape preferred orientation that have developed during solid-state deformation are identical to those of magmatic flow. This identical fabric development during solid-state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods, i.e., hornblendes have always acted as rigid particles in a viscous matrix. TEM observations show no evidence for intracrystalline plasticity in hornblende grains. The modal abundance of hornblende decreases progressively with increasing solid-state deformation to yield more biotite, epidote and quartz. Small hornblende matrix grains form by cataclastic processes. The observed changes in grain size, shape and abundance of hornblende occur mainly by fracturing, dissolution and reaction, so that the solid-state deformation of the tonalite is a complex process, to which fracturing, dissolution of hornblende and metamorphic reactions all contribute. The relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction. It is inferred that generally, hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650–750°C in the presence of an aqueous fluid. Crystal plasticity could become dominant at higher temperatures and/or lower aqueous fluid activities.