Microfabrics of omphacite and garnet in eclogite from the Lanterman Range, northern Victoria Land, Antarctica

摘要

We examined the microfabrics of omphacite and garnet in foliated eclogite to determine the influence of the layered structure on seismic observations in subduction zone. The analyzed eclogite, from the Lanterman Range, northern Victoria Land, Antarctica, is characterized by layering in which the modal abundances of garnet and omphacite vary. For garnet, the low aspect ratios, similar angular distribution of long axes relative to the foliation in both layers, uniform grain size distribution, near-random crystallographic preferred orientations (CPOs), and misorientation angle distributions are indicative of passive behavior during deformation. In contrast, omphacite shows relatively high aspect ratios, a low angle between the long axes of crystals and the foliation, a wide grain-size distribution, and distinctive CPOs, suggesting dislocation creep as the main deformation mechanism. The results of fabric analyses are consistent with strain localization into omphacite or omphacite-rich layers rather than garnet or garnet- rich layers. The single-crystal seismic anisotropy of garnet is very weak (AV(P) = 0.2%, AV(S) = 0.5-0.6%), whereas that of omphacite is much stronger (AV(P) = 3.7-5.9% and AV(S) = 2.9-3.8%). Seismic anisotropy of the omphacite-rich layers shows an increase of 329% for AV(P) and 146% for AV(S) relative to the garnet-rich layers. Our results demonstrate the importance of the layered structure in strain localization and in the development of the seismic anisotropies of subducting oceanic crust.