PETROFABRIC AND STRENGTH OF SIO2 NEAR THE QUARTZ-COESITE PHASE BOUNDARY

Quartz and coesite constitute an important portion of deep subducted continental upper crust. However, the rheology and petrofabric of quartz and coesite are not well constrained under HP/UHP metamorphic conditions both in laboratory and in nature. We report here first deformation microstructure and lattice preferred orientations of garnet, omphacite, interstitial coesite, quartz and rutile in the UHP eclogites from the Yangkou Bay, Sulu UHP terrane, as well as fabric development of quartz and coesite in shear experiments at P/T conditions near the quartz-coesite phase boundary. Our results show: 1) Garnet and coesite develop weak to random fabric while omphacite, quartz and rutile develop pronounced fabrics in deformed natural eclogites; 2) Quartz develops fabrics in responding to a dominant c-slip with increasing shear strain under HP/UHP conditions; 3) Coesite develops fabrics in responding to a dominant [100](010) slip with increasing shear strain under UHP conditions; 4) The relative strength of major constituent minerals in continental deep subduction zones are quartz < omphacite < coesite. These results are consistent with the rheology determined in the laboratory for quartz, jadeite/omphacite and coesite. We propose that jadeite/omphacite dominates the rheology and seismic anisotropy of deeply subducted continental crust.