Relationship between the strain and the activity of slip systems in deforming quartz from misorientation and shape-preferred orientation of quartz phenocrysts in a deformed granite porphyry

抄録

During plastic deformation of quartz under the upper crustal conditions, <a> slip systems are predominantly active, which has been considered to be temperature-dependent (Law 2014). However, based on a detailed textural analysis of CPOs of experimentally and naturally deformed quartz samples, Kilian and Heilbronner (2017) argued that the basal <a> slip system is not the dominant slip system under the upper crustal conditions. To clarify the relative importance of slip systems in quartz, based on optical and EBSD observations, we analyze quartz phenocrysts in deformed granite porphyry samples of different strains reported by Kano and Takagi (2013). The quartz phenocrysts exhibit undulate extinction, commonly elongate and lenticular with different aspect ratios (ARs) (i.e., strain) ranging from 1.2 to 6.3 to identify the dominant active slip system based on misorientation analyses via EBSD data (Lloyd et al. 1997). The misorientation axis of almost all concentrated subparallel to the Y-axis (i.e., rotation axis) of the sample coordinates. Only the phenocryst with AR=1.8 has concentrated in the periphery near the Z-axis of the sample coordinates and also parallel to the [c] axis; the high density distributed around the [c] axis in the crystal coordinates may be twist boundary. Phenocrysts with ARs of 6.3 and 3.7, the misorientation density shows a maximum around the [c] axis, indicative of the activity of prism <a>. Phenocryst with AR of 3.1, the highest density concentration is close to the {m} and perpendicular to the [c] axis, indicative of the activity of basal <a> (+ prism [c]). The phenocryst with AR of 1.6 has distributed two concentrations around the [c] and <a> axes, indicative of twist boundary (basal +basal ) and the activity of prism [c]. Consequently, the dominant slip systems in the quartz phenocrysts are prism <a> and basal <a> (+ prism [c]). The values of the AR of quartz phenocrysts indicative of the activity of prism <a> slip system are higher than those with basal <a> slip system, implying that the CRSS for the former is lower than that for the latter. References: Kano and Takagi (2013) J Geol Soc Japan 119:776. Lloyd et al. (1997) Tectonicphysics 279:55. Kilian and Heilbronner (2017) Solid Earth 8:1095. Law (2014) J Struct Geol 66:129.