Mechanisms for the nucleation and deformation of symplectites during omphacite breakdown     

Abstract

The breakdown of omphacite is one of the first signs of eclogite retrogression, and typically results in the formation of vermicular intergrowths of sodic plagioclase and diopside (± quartz ± amphibole), termed clinopyroxene-plagioclase symplectites. Such symplectites occur in most, if not all, eclogite localities worldwide. The reaction is associated with a substantial grain size reduction, and may thus significantly impact bulk rock rheology during eclogite exhumation. We study a suite of natural clinopyroxene-plagioclase symplectites by electron backscatter diffraction (EBSD). The sample suite comprises symplectites in various stages of their evolution: the beginning stages of nucleation (vermicular symplectites), partially recrystallized symplectites, and completely recrystallized and strongly deformed symplectites. We determine crystallographic relationship between the parent omphacite and the reaction products and interphase misorientation relationships between the reaction products (plagioclase and diopside), to shed light on nucleation and deformation mechanism during eclogite retrogression. We find that the nucleation of diopside and plagioclase in the symplectites is strongly controlled by the crystallography of the parent omphacite, with the diopside copying the crystal lattice of the parent grain, and the plagioclase nucleating in special orientation relationships to the diopside, along planes with favorable interplanar spacing. Initially strong crystallographic relationships are weakened as deformation of the symplectites proceeds by fracturing transitioning into grain boundary sliding accommodated by diffusion creep, i.e., grain-size sensitive (GSS) creep.The results indicate that the formation of clinopyroxene-plagioclase symplectites does not increase permeability in crustal rocks, initially, but that deformation by GSS creep leads to progressive hydration and weakening of eclogites during retrogression. The symplectites thus significantly impact bulk crustal rheology.