P- (Vp) and S-wave (Vs) velocities of polycrystalline plagioclase (An 51±1) and gabbronorite including plagioclase (An 49±2) (62.3vol.%) were measured up to 700°C at 0.6, 0.8, and 1.0GPa during heating and cooling. Both polycrystalline plagioclase and gabbronorite show reversible and discontinuous change in temperature derivative of Vp and Vs, ∂Vp/∂T and ∂Vs/∂T, respectively, at about 400°C. The ∂Vp/∂T and ∂Vs/T of the polycrystalline plagioclase changes from -0.8 × 10-4 to -3.4 × 10 -4 km s-1o C-1 and from -1.1 × 10 -4 to -3.3 × 10-4 km s-1o C-1, respectively. The ∂Vp/∂T and ∂Vs/∂T of the gabbronorite varies from -1.0 × 10-4 to -3.4 × 10-4 km s-1o C-1 and from -0.4 × 10-4 to -3.5 × 10-4 km s-IoC-1, respectively. The reversible and discontinuous changes in ∂Vp/∂T and ∂Vs/∂T are attributed to a phase transformation of plagioclase. The present data suggest that the sharp decrease in Vp and Vs would cause low velocity anomaly under dry and subsolidus melt-absent conditions in the mid-to-lower crust of relatively high heat flow regions. Copyright 2006 by the American Geophysical Union.