HR: 08:05h
AN: V51B-01 INVITED     [Abstracts]
TI: On the Deformation of UHP Eclogite: From Laboratory to Nature
AU: * Zhang, J
EM: junfeng.zhang@ucr.edu
AF: IGPP, Univ. of California, Riverside, CA 92521, United States
AU: Green, H W
EM: harry.green@ucr.edu
AF: IGPP, Univ. of California, Riverside, CA 92521, United States
AU: Green, H W
EM: harry.green@ucr.edu
AF: Dept. of Earth Sciences, Univ. of California, Riverside, CA 92521, United States
AB: Eclogites from orogens often show strong plastic deformation together with the surrounding country rocks represented by felsic gneisses. Eclogite rheology is an important piece of information for understanding the subduction of continental crust to great depth and then return to the surface. In the past two decades, there have been numerous studies of naturally deformed eclogites using a combination of optical microscopy, scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Most of these studies suggest that omphacite accommodates most of the strain imposed on eclogite and develops strong lattice preferred orientations (LPOs); garnet crystals generally behave as rigid grains. However, there have been some recent controversies regarding the ductility of garnet and LPO development in omphacite. Natural garnets deformed by dislocation creep and by grain boundary processes have both been proposed based on a variety of observations and other evidence. The variation of S-type and L-type fabric in omphacite has been attributed to drastically different mechanisms, such as dislocation creep, diffusion-assisted grain growth and space-group transformation. We present here the comparison side by side of deformation microstructures, fabrics, and seismic properties between naturally deformed and experimentally deformed eclogites synthesized from the same natural rocks. Our results suggest remarkable similarities despite the large difference in conditions (e.g. strain rates, stress, temperature, etc.) responsible for producing them. We conclude that: 1) In dry eclogite, omphacites accommodate most of the deformation while garnets behave as essentially rigid bodies; 2) In wet eclogite, the deformation is accommodated by shape change in both garnet and omphacite; 3) In both cases, omphacites develop pronounced S-type or L-type microfabrics arising from dislocation creep and variation of the geometry and orientation of the finite strain ellipsoid; 4) Garnets develop no LPO in either case; 5) The seismic properties of eclogites are controlled by both garnet and omphacite but their anisotropy patterns of P-wave velocity are controlled only by the LPO of omphacite.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 8030 Microstructures
DE: 8032 Rheology: general (8160)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly