TEXTURAL AND MECHANICAL EVOLUTION WITH PROGRESSIVE STRAIN IN EXPERIMENTALLY DEFORMED APLITE

Determining meaningful flow laws for the polyphase aggregates that cha racterize the crust is difficult, because the strength of such aggrega tes depends on the strengths, volume proportions, and geometrical arra ngement of their constituent phases, and all of those factors may chan ge with progressive strain. In order to investigate some of these fact ors, we have performed a series of axial compression experiments on a fine-grained granite (aplite) over a range of shortening strains (20-8 5%) and temperatures (700-900 degrees C) at 1.5 X 10(-6) s(-1) and 140 0 MPa; we have also done comparison experiments on pure quartz and fel dspar aggregates. The aplite is composed of similar to 1/3 quartz and 2/3 oligoclase and microcline, with minor biotite and magnetite; the q uartz grains are dispersed in a continuous matrix of feldspar. Optical - and TEM-scale microstructures indicate that the deformation mechanis ms of the quartz and feldspar grains in the aplites are similar to tho se of pure quartz and feldspar aggregates deformed at the same conditi ons. The feldspars deform by semi-brittle flow at 700 and 800 degrees C and by recrystallization-accommodated dislocation creep at 900 degre es C, while quartz deforms by recrystallization-accommodated dislocati on creep at 700 degrees C and by climb-accommodated dislocation creep at 800 and 900 degrees C. Pure albite aggregates are stronger than qua rtz aggregates over this range of conditions; however, in the aplite a n apparent strength reversal is inferred from grain strain measurement s and changes in phase distribution with increasing temperature. At 70 0 degrees C, the dispersed quartz grains in the aplite remain less def ormed than the feldspar grains because the scarcity of quartz-quartz g rain boundaries limits recovery by grain boundary migration recrystall ization, causing the quartz strength to rise above that of feldspar. H owever, the aplite is weaker than a pure feldspar aggregate because cr acking is enhanced by stress concentrations at the quartz grains. At 8 00 degrees C, the two phases undergo approximately equal strain becaus e 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 degrees C, the initially continuous feldspar grains deform inhomogeneously and undergo microboudinage, while the quartz g rains deform homogeneously and gradually become more interconnected an d undergo higher strain than the feldspars; the aplite strength is ini tially closer to that of a feldspar aggregate but with increasing stra in it becomes closer to that of a quartz aggregate. The aplite undergo es strain softening over the whole range of conditions. At 700 and 800 degrees C, the weakening is due to grain boundary migration recrystal lization of feldspar in the fine-grained crush products along grain-sc ale faults; at 900 degrees C, the weakening is due to a combination of grain boundary migration recrystallization of the feldspars plus incr easing interconnection of the weaker quartz grains. These results demo nstrate that the textural evolution of a polyphase aggregate with incr easing strain is determined by both the phase distribution and the def ormation mechanism of each phase and that very high strains may be req uired to approach microstructural and mechanical steady state, Therefo re, measuring or calculating a flow law appropriate for 'granite' at m id-crustal conditions is not simple.