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TitleA new calibration of seismic velocities, anisotropy, fabrics, and elastic moduli of amphibole-rich rocks
AuthorJi, S CORCID logo; Shao, T BORCID logo; Michibayashi, KORCID logo; Long, C X; Wang, Q; Kondo, Y; Zhao, W H; Wang, H C; Salisbury, M H
SourceJournal of Geophysical Research, Solid Earth vol. 118, 9, 2013 p. 4699-4728, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200239
PublisherAmerican Geophysical Union
Mediapaper; on-line; digital
File formatpdf
Subjectsgeochemistry; Science and Technology; tectonics; igneous and metamorphic petrology; seismic velocities; anisotropy; fabric analyses; amphibolites; continental crust; oceanic crust
Illustrationstables; distribution diagrams; graphs; equal-area stereonet projections; diagrams; plots
Released2013 08 29
AbstractA large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase. We have measured P and S wave velocities (V-p and V-s) and anisotropy of 17 amphibole-rich rock samples containing 34-80vol% amphibole at hydrostatic pressures (P) up to 650MPa. Combined petrophysical and geochemical analyses provide a new calibration for mean density, average major element contents, mean V-p-P and V-s-P coefficients, intrinsic V-p and V-s anisotropy, Poisson's ratios, the logarithmic ratio R-s/p, and elastic moduli of amphibole-rich rocks. The V-p values decrease with increasing SiO2 and Na2O+K2O contents but increase with increasing MgO and CaO contents. The maximum (0.38-0.40km/s) and minimum S wave birefringence values occur generally in the propagation direction parallel to Y and normal to foliation, respectively. Amphibole plays a critical role in the formation of seismic anisotropy, whereas the presence of plagioclase, quartz, pyroxene, and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations (CPOs). The CPO variations cause different anisotropy patterns illustrated in the Flinn diagram of V-p(X)/V-p(Y)-V-p(Y)/V-p(Z) plots. The results make it possible to distinguish, in terms of seismic properties, the amphibolites from other categories of lithology such as granite-granodiorite, diorite, gabbro-diabase, felsic gneiss, mafic gneiss, eclogite, and peridotite within the Earth's crust. Hence, amphibole, aligned by dislocation creep, anisotropic growth, or rigid-body rotation, is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs, which contains rather little phyllosilicates such as mica or chlorite.

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