We investigated plastic flow of synthetic two-phase aggregates composed of anorthite and 0--75 wt % quartz. The aggregates deform predominantly by grain boundary diffusion creep at temperatures of 1173--1473 K, strain rates of 10-7--10-4 s-1, and at confining pressure of 0.1 and 300 MPa. Strength of the aggregates increases with increasing content of quartz particles that behave as almost rigid inclusions. Strengthening is more pronounced for samples deformed at 0.1 MPa than at 300 MPa confining pressure. Predictions of composite strength based on self-consistent continuum models are in good agreement with the experimental data from uniaxial experiments at quartz fractions 5000 H/106Si at 300 MPa confining pressure in agreement with observed microstructures. Transmission electron microscope observations indicate local gradients in deformation around quartz particles dominantly in samples deformed at atmospheric pressure.