On the geometric relationship between deformation microstructures in zircon and the kinematic framework of the shear zone

Abstract

We present novel microstructural analyses of zircon from a variety of strained rocks. For the first time, multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones. Our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation, based on careful in situ observations. For sampling, we selected zircon-bearing rocks that were deformed by simple shear. Samples covered a range of P–T conditions and lithologies, including various meta-igneous and meta-sedimentary gneisses.Microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of: (i) the long axes of plastically deformed zircon crystals, (ii) the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and (iii) the foliation and lineation directions of the respective samples. We assume that zircon crystals did not experience post-deformation rigid body rotation, and thus the true geometric link can be observed. The relationships are the following: (a) plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane; (b) crystals with <c> axes oriented at an angle>15° to the foliation plane are undeformed or fractured.Zircon crystals that have <c> axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the [001] crystallographic direction. Zircon grains with the <c> axis aligned at 30–60° to the lineation within the foliation plane often develop either two low Miller indices misorientation axes or one high Miller indices misorientation axis. Host phases have a significant influence on deformation mechanisms. In a relatively soft matrix, zircon is more likely to develop low Miller indices misorientation axes than in a relatively strong matrix.These relationships are independent of zircon's grain size and shape, and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal-plastic deformation behavior of zircon and on the geometry of its slip systems. We describe previously unknown rheological and crystallographic properties of zircon, which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation. Additionally, relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology. If deformation events result in zircon distortion and, as a consequence, partial or complete resetting of the zircon isotopic system, the age of deformation can be accurately dated.