Crystallographic preferred orientations of plagioclase via grain boundary sliding in a lower-crustal anorthositic ultramylonite
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To clarify the rheological properties and deformational mechanisms that operate within lower-crustal shear zones, we analyzed deformation microstructures and fabrics in plagioclase grains from high-strain regions of intrafolial drag folds of an anorthositic ultramylonite from Langøya, Vesterålen, northern Norway. These grains have developed weak crystallographic preferred orientations (CPOs). Other than deformation twinning, they do not show any indications of intracrystalline plasticity or dynamic recrystallization. In some domains of the drag fold, plagioclase CPOs are more distinct, but the majority of inferred slip systems are not consistent with the main slip systems in plagioclase. These suggest that neither dislocation creep nor dislocation-accommodated grain boundary sliding (GBS) produced the plagioclase CPOs but rather grain rotation via GBS along specific grain boundaries parallel or subparallel to crystallographic planes of (010), (100) or (110), which led to an alignment of the easy-slip grain boundaries in the flow direction that produced the observed CPO patterns. The anisotropy of dissolution and growth rates may have been responsible for the development of the specific grain boundaries.
KeywordsPlagioclase Anorthositic mylonite Lower crust Crystallographic preferred orientation Grain boundary sliding
We thank N. Shigematsu for granting us permission to use the SEM–EBSD at the Geological Survey of Japan, AIST, and K. Ishii, J. Gómez Barreiro, and J. Fukuda for fruitful discussions. Nearly all the MTEX scripts used in this study were originally written by D. Mainprice. This paper benefited from detailed reviews by two anonymous referees. S. Mukherjee is thanked for his editorial handling of the manuscript. This study was supported in part by MEXT KAKENHI Grant Numbers 26109004, 16K05613, and 18H01297.
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