Finite lattice distortion patterns in plastically deformed zircon grains

Abstract

Abstract. This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, Eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea-Verbano Zone (IVZ), Southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are: granitic orthogneisses, meta-lamprophyres and paragneisses, all of which are highly deformed. The investigated zircon grains ranging from 10 to 50 microns were studied in situ using a combination of scanning electron microscope (SEM) techniques, including secondary electron (SE), backscattered electron (BSE), forward scattered electron (FSE), cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction analysis (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: Type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6° to 1.4° per μm without subgrain boundary formation. Type (II) represents local gradual bending of the crystal lattice coupled with the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 7° to 40° within single grains. Type (III) is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour in 2-D sections. The cumulative orientation variation within a single grain ranges from 3° to 10°. Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. Investigated microstructures demonstrate that misorientation axes are usually parallel to the ⟨ 001 ⟩ and ⟨ 100 ⟩ crystallographic directions; dominant slip systems operating along tilt boundaries are ⟨ 010 ⟩{001}, ⟨ 010 ⟩{100} and ⟨ 001 ⟩{010}. In case of twist boundaries the slip systems ⟨ 010 ⟩{001} and ⟨ 100 ⟩ {001} are active, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is mostly controlled by the degree of coupling with the host phase and by the viscosity ratio between inclusion and host, and defined by crystallographic and elastic anisotropy of the zircon lattice.