Textural and mechanical evolution with progressive strain in experimentally deformed aplite

Abstract

Determining meaningful flow laws for the polyphase aggregates that characterize the crust is difficult, because the strength of such aggregates depends on the strengths, volume proportions, and geometrical arrangement of their constituent phases, and all of those factors may change with progressive strain. In order to investigate some of these factors, we have performed a series of axial compression experiments on a fine-grained granite (aplite) over a range of shortening strains (20–85%) and temperatures (700–900°C) at 1.5 × 10−6 s−1 and 1400 MPa; we have also done comparison experiments on pure quartz and feldspar aggregates. The aplite is composed of ∼1/3 quartz and 2/3 oligoclase and microcline, with minor biotite and magnetite; the quartz grains are dispersed in a continuous matrix of feldspar. Optical- and TEM-scale microstructures indicate that the deformation mechanisms of the quartz and feldspar grains in the aplites are similar to those of pure quartz and feldspar aggregates deformed at the same conditions.The feldspars deform by semi-brittle flow at 700 and 800°C and by recrystallization-accommodated dislocation creep at 900°C, while quartz deforms by recrystallization-accommodated dislocation creep at 700°C and by climb-accommodated dislocation creep at 800 and 900°C. Pure albite aggregates are stronger than quartz aggregates over this range of conditions; however, in the aplite an apparent strength reversal is inferred from grain strain measurements and changes in phase distribution with increasing temperature. At 700°C, the dispersed quartz grains in the aplite remain less deformed than the feldspar grains because the scarcity of quartz-quartz grain boundaries limits recovery by grain boundary migration recrystallization, causing the quartz strength to rise above that of feldspar. However, the aplite is weaker than a pure feldspar aggregate because cracking is enhanced by stress concentrations at the quartz grains. At 800°C, the two phases undergo approximately equal strain because easy dislocation climb in quartz allows individual grains to strain homogeneously; the aplite strength is close to that of a pure feldspar aggregate. At 900°C, the initially continuous feldspar grains deform inhomogeneously and undergo microboudinage, while the quartz grains deform homogeneously and gradually become more interconnected and undergo higher strain than the feldspars; the aplite strength is initially closer to that of a feldspar aggregate but with increasing strain it becomes closer to that of a quartz aggregate. The aplite undergoes strain softening over the whole range of conditions. At 700 and 800°C, the weakening is due to grain boundary migration recrystallization of feldspar in the fine-grained crush products along grain-scale faults; at 900°C, the weakening is due to a combination of grain boundary migration recrystallization of the feldspars plus increasing interconnection of the weaker quartz grains. These results demonstrate that the textural evolution of a polyphase aggregate with increasing strain is determined by both the phase distribution and the deformation mechanism of each phase and that very high strains may be required to approach microstructural and mechanical steady state. Therefore, measuring or calculating a flow law appropriate for ‘granite’ at mid-crustal conditions is not simple.