Seismic Velocities, Anisotropy and Petrofabrics of Amphibolite from the Gaoligong Mts., Yunnan
详细信息
- 责任者：Ji Shaocheng (Département des Génies Civil ; Géologique et des Mines ; cole Polytechnique de Montréal ; Montréal ; Québec ; H3C 3A7 ; Canada) ; Michibayashi Katsuyoshi
- 刊名：Geological Review
- 出版年：2013
- 卷期：59(4)
- 关键词：角闪岩 ; amphibolites ; 弹性波 ; elastic waves ; 各向异性 ; anisotropy ; 高黎贡山 ; Gaoligong Mountains
- 页码：p.769-780
- 图表：illus., 2 tables, 53 refs.
- 分类号：0802
- issnNo：ISSN0371-5736
- isbnNo：CN11-1952/P
摘要
A large portion of the deep crust beneath the continents and oceanic island arcs may consist of amphibolites dominated by hornblende and plagioclase. An amphibolite can be derived from prograde metamorphism of either mafic igneous rocks or greywackes, or retrograde metamorphism of eclogites and mafic granulites. Furthermore, partial melting of amphibolitic lower crust or subducted oceanic crust is the most likely source for the formation of plutonic bodies of granites, migmatites and trondhjemite-tonalites during orogenic events. Here we report new results on P- and S-wave velocities and anisotropy in a typical amphibolite from the Gaoligong ductile shear zone, Western Yunnan, China, measured along 7 key directions at confining pressures up to 600 MPa. At 600 MPa, P- and S-wave anisotropy coefficients are 14.5% and 12.0%, respectively. The maximum shear wave splitting (0.38 km/s) occurs in the propagating direction at an angle of 55° with each of the three main axes of the finite strain ellipsoid (X is parallel to the stretching lineation, Y parallel to the foliation but perpendicular to the lineation, and Z normal to the foliation). To the first approximation, the spatial distribution of P-wave velocities in the amphibolites can be represented by an ellipsoid whose longest, moderate and shortest axes are parallel to the X, Y and Z axes of the strain ellipsoid, respectively. Electron backscattering diffractions (EBSD) showed that the (100) planes of hornblende are parallel or subparallel to the XY plane, the [001] crystallographic directions have a strong concentration parallel to X and the [010] directions are parallel to Y. This petrofabric pattern suggests that the hornblende deformed by (100)[001] slip accommodated by anisotropic growth. The seismic anisotropy and shear wave splitting can be well explained by the lattice preferred orientations of hornblende (87 vol%), plagioclase (7 vol%) and quartz (6 vol%), characterized using EBSD techniques. The results provide a new calibration for the seismic properties of amphibolites within the Earth’s crust in order to improve our further understanding of crustal deformation and tectonic evolution.
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