MICROSTRUCTURAL EVIDENCE OF DISLOCATION CREEP AND DIFFUSION ACCOMMODATED DEFORMATION OF GLAUCOPHANE IN A NATURALLY DEFORMED LAWSONITE BLUESCHIST

Blueschist rheology is crucial to the mechanical behavior of subducting oceanic crust. Mafic blueschists are often ubiquitous along the plate interface from the base of the seismogenic zone to sub-arc depths, yet the lithology strength remains poorly constrained. Observations from exhumed subduction terranes suggest that blueschist accommodates significant strain, largely partitioned into the sodic amphibole glaucophane (Gln). However, it remains an open question whether the observed deformation is accommodated by dislocation or diffusion related processes.

We investigated the Gln fabric and deformation mechanisms in a garnet-bearing lawsonite blueschist (LBS) block from the mélange unit of the Catalina Schist subduction complex on Pimu’nga. This block records eclogite facies peak conditions with an LBS-facies overprint. We used electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to interpret the textural and geochemical record of deformation during subduction. Microstructural analysis of the Gln reveals evidence of dislocation accommodated deformation including: (1) crystallographic preferred orientation (CPO) development, (2) intragranular misorientations, (3) dislocation motion on multiple slip systems, and (4) subgrain boundary formation. Evidence for inherited CPOs in core-mantle structures imply that subgrain boundary recrystallization was active. This microstructural evidence suggests that dislocation-creep-accommodated deformation was active. SEM and EDS maps of Gln reveals evidence of chemical zoning with higher Fe and lower Al and Mg concentrations along microfractures suggesting a fluid-mediated/diffusion-accommodated deformation process such as micro-boudinage and/or coupled dissolution-precipitation. Titanite in the matrix and as inclusions in texturally equilibrated Gln and lawsonite grains signals significant Gln growth and deformation occurred post-peak conditions at lawsonite blueschist conditions. These observations are supported by our pseudosection modelling results for the garnet blueschist block. Together, these results suggest that Gln can readily deform by dislocation creep, and also record diffusion-mediated processes during deformation under lawsonite blueschist facies conditions.