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Durham University

Department of Earth Sciences

Profile

Publication details for Professor Yaoling Niu

Zhao, Z.D., Niu, Yaoling, Christensen, N.I., Zhou, W., Hou, Q., Zhang, Z.M., Xie, H., Zhang, Z.C. & Liu, J. (2011). Delamination and ultra-deep subduction of continental crust: constraints from elastic wave velocity and density measurement in ultrahigh-pressure metamorphic rocks. Journal of Metamorphic Geology 29(7): 781-801.

Author(s) from Durham

Abstract

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 (Vp) and shear wave (Vs) velocities in three mutually perpendicular directions under hydrostatic pressures up to 1.0 GPa were measured for each sample. At 1.0 GPa, Vp (7.5–8.4 km s−1), Vs (4.2–4.8 km s−1), and densities (3.2–3.6 g cm−3) in the UHP eclogites are higher than those of UHP orthopyroxenite (7.3–7.5 km s−1, 4.1–4.3 km s−1, 3.2–3.3 g cm−3, respectively) and HP eclogites (7.1–7.9 km s−1, 4.0–4.5 km s−1, 3.1–3.5 g cm−3, respectively). Kyanitites (with 99.5% kyanite) show extremely high velocities and density (9.37 km s−1, 5.437 km s−1, 3.581 g cm−3, respectively). The eclogites show variation of Vp- and Vs-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 253 GPa, respectively. The Vp and density of representative UHP metamorphic rocks (eclogite & kyanitite) were extrapolated to mantle depth (15 GPa) following a reasonable geotherm, and compared to the one dimension mantle velocity and density model. The comparison shows that Vp and density in eclogite and kyanitite are greater than those of the ambient mantle, with differences of up to ΔVp > 0.3 km s−1 and Δρ > 0.3–0.4 g cm−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.