【摘要】 斜长角闪岩和角闪岩相变质岩石是大陆中—下地壳、岛弧深部地壳以及俯冲大洋地壳中最重要的组成岩石之一,查明目前地壳中斜长角闪岩的体积含量、空间分布及其应变状态对于深入研究大陆地壳的形成与演化过程极其重要。笔者等实验测量了云南高黎贡韧性剪切带典型变形斜长角闪岩7个关键性方向上的地震(P和S波)波速随静水围压(0～600MPa)的变化规律,并利用电子背散射衍射(EBSD)技术测定了主要造岩矿物角闪石、斜长石和石英的晶格优选方位,查清斜长角闪岩中地震波速各向异性与剪切波分裂的成因,确定斜长角闪岩中有限应变椭球与波速椭球的对应关系,为今后利用原地地震波速的各向异性调查地壳深部区域构造应变场提供必要的实验和理论基础。更多还原

【Abstract】 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.更多还原