Elastic wave velocity in rocks from Dabieshan and its constraints for lithospheric composition and crust-mantle recycling

摘要

P- and S-wave velocities in eclogites and granulites from the Dabie ultrahigh pressures (UHP) meta- morphic belt, China, were measured at room temperature under the hydrostatic pressures up to 1.0 Gpa. The ultrahigh pressure eclogites had the highest densities (3.3-3.6 g.cm~-3), high velocities and the lowest anisotropy (1.4/100- 2.6/100). The lowest densities (2.8-3.1 .cm~-3) and the highest Poisson's rations (0.28-0.29) were found in gran- ulites, whereas the strongest anisotropies(6.1-8.4/100) were found in the high-pressure (HP) eclogites. Compari- son of the velocities in rocks with that observed in the deep seismic sounding profile crossing Dabieshan suggests that e- clogites might exist in the lower crust of Dabieshan, but the quantity might be small. The upper mantle has very similar velocities as the UHP eclogites and serpentinizated/water-bearing dunite. The formation of eclogite represents the crust- mantle recycling processes. Crustal material is delaminated and sinks into the mantle by way of eclogite, whereas only a small part of the eclogite could return to the crust