Delamination and ultra-deep subduction of continental crust: Constraints from elastic wave velocity and density measurement in ultrahigh-pressure metamorphic rocks

摘要

Thirty-three samples, including 22 eclogites, collected from the Dabie ultrahigh-pressure (UHP) metamorphic belt in eastern China, have been studied for seismic properties. Compressional (V_p) and shear wave (V_s) velocities in three mutually perpendicular directions under hydrostatic pressures up to 1.0GPa were measured for each sample. At 1.0GPa, V_p (7.5-8.4kms~(-1)), V_s (4.2-4.8kms~(-1)), and densities (3.2-3.6gcm~(-3)) in the UHP eclogites are higher than those of UHP orthopyroxenite (7.3-7.5kms~(-1), 4.1-4.3kms~(-1), 3.2-3.3gcm~(-3), respectively) and HP eclogites (7.1-7.9kms~(-1), 4.0-4.5kms~(-1), 3.1-3.5gcm~(-3), respectively). Kyanitites (with 99.5% kyanite) show extremely high velocities and density (9.37kms~(-1), 5.437kms~(-1), 3.581gcm~(-3), respectively). The eclogites show variation of V_p- and V_s-anisotropy up to 9.70% and 9.17%, respectively. Poisson's ratio (σ) ranges from 0.218 to 0.278 (with a mean of 0.255) for eclogites, 0.281-0.298 for granulites and 0.248 to 0.255 for amphibolites. The σ values for serpentinite (0.341) and marble (0.321) are higher than for other lithologies. The elastic moduli K, G, E of kyanitite were obtained as 163, 102 and 253GPa, respectively. The V_p and density of representative UHP metamorphic rocks (eclogite & kyanitite) were extrapolated to mantle depth (15GPa) following a reasonable geotherm, and compared to the one dimension mantle velocity and density model. The comparison shows that V_p and density in eclogite and kyanitite are greater than those of the ambient mantle, with differences of up to ΔV_p>0.3kms~(-1) and Δρ>0.3-0.4gcm~(-3), respectively. This result favours the density-induced delamination model and also provides evidence in support of distinguishing subducted high velocity materials in the upper mantle by means of seismic tomography. Such ultra-deep subduction and delamination processes have been recognized by seismic tomography and geochemical tracing in the postcollisional magmatism in the Dabie region.