Connection between chemical zonation and crystallographic preferred orientation as an indicator for the fabric development in eclogites. Example from the Saxothuringian eclogites, Bohemian massif, Czech Republic

Abstract

The presence of eclogites is one of the strongest evidences for high pressure metamorphism, and for the presence of a paleo subduction zone. Deformation of eclogites can yield important information about the kinematics of subduction zones. The main mechanism for fabric development in eclogites is still a matter of debate among various research groups and has split the field in two. Most previous studies suggest that dislocation creep is the main deformation mechanism, but some studies suggest grain boundary diffusion as the main mechanism for the elongation of the high-pressure minerals and fabric evolution.

Eclogites from the Saxothuringian domain in the Czech part of the Erzgebirge in the Bohemian Massif exhibit a strong lineation fabric. Omphacite, zoisite and elongated garnet contribute most to this fabric. SEM, BSE, EDS and EBSD analysis of the eclogites show an asymmetric, prograde zonation in omphacite and garnet, which is related to the elongation direction, parallel to lineation. Zoisite is expected to act as a rigid particle without any intracrystalline deformation, yet it occurs as elongated particles in the lineation direction. Quartz is located as elongated aggregates in pressure shadows of garnet parallel to lineation and does not show any signs of intra crystalline deformation.

The asymmetric prograde chemical zonation of omphacite and garnet indicates growth and elongation of garnet and omphacite during prograde and peak metamorphic conditions. The correlation of the zonation patterns in omphacite and garnet, as well as weak CPO in garnet suggest that the elongated shape of the minerals and the fabric in the eclogites are a result of dissolution and grain growth by grain boundary diffusion. As well as rotation and oriented dependent growth of omphacite in the [001] direction causing a strong CPO and elongation in the lineation direction.