Microstructural analysis of zoisite from a naturally deformed eclogite (Syros, Greece): Implications on the rheology of metabasic rocks along the subduction zone interface

Subducted metabasic rocks play an important role in defining the rheology of the subduction zone interface and are believed to be the primary source of volatiles for arc magmatism and fluid-induced seismicity. The rheology of metabasic rocks along the subduction zone interface are typically modeled based on the strength of omphacite, however metabasic rocks are made up of a variety of different minerals. In this study we present a detailed microstructural analysis of zoisite from an eclogite collected on Kini beach in Syros, Greece and make the argument that

 zoisite may be important in defining the rheology of metabasic rocks along the subduction zone interface.The studied sample is a prograde blueschist to eclogite facies rock primarily containing glaucophane, omphacite, zoisite, and garnet with minor amounts of chlorite retrogression at the rims of large omphacite grains. A hand sample (~12 cm in diameter) was polished and several thin sections were made for analyses. The hand sample shows strain is primarily localized into zoisite-rich layers suggesting zoisite was the weakest mineral when the sample was deforming. Backscatter electron images of the zoisite layers show a homogeneous mixture of zoisite and clinozoisite, where clinozoisite makes up ~10% of the zoisite layers. Zoisite grains are on average twice as large as clinozoisite with an average grain size of 61 μm, whereas clinozoisite has an average grain size of 36 μm. Both phases have strong shape and crystallographic preferred orientations. Both phases show poles to (100) maxima roughly orthogonal to the reference foliation, while poles to (010) develop a girdle sub-parallel to the foliation plane, and poles to (001) are uniformly distributed. EBSD analysis shows that a few zoisite and clinozoisite grains develop subgrain boundaries, suggesting dislocation related mechanisms played a minor role. Based on our analysis, zoisite is interpreted to have deformed primarily by a combination of rigid body rotation and dissolution-precipitation, which lead to the precipitation of clinozoisite. Further analyses will be conducted on zoisite as well as other common minerals within the sample.